IN A CORNER of the glass case, a mouse is washing its paws. Its pink tongue dips and flickers across miniature wrists and bonsai arms, while its pink nose burrows in its snow-white, three-day-old baby fur - a sight to charm the hardest heart - as it crouches in a little rush nest, oblivious to everything else.

Oblivious, for instance to the 20 pairs of eyes that are regarding it with horror through the glass wall before us. For we are watching the mouse's bathroom ritual like movie-goers in a nasty suspense film. We know, as it clearly does not, what is about to happen. We can see, not six inches from the mouse's makeshift lavabo, that two of the most enormous and evil snakes in South America are lying curled up asleep like moire firehoses and the mouse is just seconds away from becoming their lunch.

"Ees a Cascaval," says my guide from the tourist office, "ver' ver' nasty. 'E has the, 'ow you say, rattle in the tail." The sign above the glass case gives the snake's Latin name as crotalus durissimus terrificus, which makes it sound a real hard case. (The poor mouse, by contrast is just a mus infantus terriblysweetus).

The 17 Rio schoolchildren, their laughing lady teacher, the tourist guide and I regard the scene with varying degrees of astonishment. How can they, in a modern municipal zoo, allow children to watch a reptilian predator devour a live mouse right before their eyes? Is this a symbol of some ingrained barbarism that lurks in modern Brazil, no matter how sophisticated its economy, how classy its football, how radical its recent image-burnish? No it's not. It's just typical Rio common sense: show the kids the snakes, but show them, simultaneously, what snakes feed on. A brisk learning curve in the amorality of nature.

The mouse concludes its toilette and goes for a stroll across the bigger snake's bulging hind quarters. The mouse's days, even its seconds are numbered. I can't look. The schoolchildren, however, do not seem bothered, they even raise a small cheer. I discover, emerging from behind my latticed fingers, that the mouse has strolled blithely over both snoozing reptiles and reached the top of a hollow tree in the corner of the cage, where he is noseing around, making tiny god-I'm-hungry gestures. The snakes are too bloated to go near him just yet. They sleep on. Natural selection is suspended for an hour or two. I make a dive for the exit.

THIS FAR south of the equator, spring is scheduled to arrive on 21 September; so technically, I'm here in Rio at the end of winter. On my first day the sun hammered down, 35C in the shade, and shimmered milkily over the Sugar Loaf, the great fat hill (rather more coffee bean than loaf) that rears so picturesquely out of the bay. Today, however, rain is pocking the brown puddles in the streets of Santa Teresa as I sit in this restaurant eating my way through mountains of roast pork with manioc and sweet potatoes and reflecting that, within a mile from here, you could encounter the massive statue of Christ the Redeemer, and the Swiss Consulate and Ronnie Biggs.

Hardly anyone swims these days in the Atlantic off Copacabana Beach or off the equally long and miraculously white Ipanema sand, but the Cariocas, as Rio dwellers are known, promenade up and down them ceaselessly. They are very proud of their beautiful, polycultural, omni-racial home. They will point out how logically Rio decided to make the whole length of Avenida Atlantica (the equivalent of the Corniche at Nice) a one-way street for three hours each weekday morning to get all the white-collar traffic off to work downtown. How logically they arranged to make all museums, galleries and means of public transport free to any children in school uniform. How kindly they decreed that no one over 65 should ever have to stand in a queue.

And Rio's street vendors have a logical insouciance about them, even when selling death. Among the rag-tag swarm of local boys selling TV aerials, batteries, key-rings and glue in the heart of the central district, there's an upturned crate on which lie four tiny jars beside a sign: "Mata e seca o rato." "It means, `Kills and dries up rats'," my guide told me, "but if the police come, 'e run away." Because rat poison is illegal, I asked? "Because too many people buy for suicide," was the reply.

A CONSTANT surprise in shops is the letter X on hundreds of price tags, denoting that you can, if you like, pay for an object in three or five or 20 instalments. This wouldn't raise an eyebrow if it were a television or hi-fi - but here it applies if you're buying a pair of socks. Why? It's the legacy of 30 years of funny money. Back in the Sixties, the Brazilian currency was the cruzeiro, and you knew where you were. In the mid-Seventies, you got the conto, and over the next 20 years the currency was devalued five times: the cruzeiro again, then the novo cruzeiro, the cruzado, then the novo cruzado ... for a time, inflation ran at 80 per cent a month; sometimes you'd need millions of whatever-they-were-called just to buy some cigarettes. The weary middle classes, stunned by successive devaluations, put their savings in special index bank accounts called "Unidad Real di Valo" - meaning "real unit of value" - and after a time realised that, logically, they might as well make the nice, stable URV the national currency. So the Real was born. And hence the city's current fascination for buying everything, even your underwear, on the never-never. It's the symbol of a people who, after 30 years, feel at last they can predict the financial future.

THE HOT news around here concerns the armed robbery that filled the middle of town with gunfire on Thursday. Three people were shot including one of the robbers. The supposed ringleader was taken to the police station and interrogated. According to the newspapers, his mobile phone rang every 10 minutes or so, as his concerned-but- stupid accomplices called to check that he was OK. The cops made a note of every phone call, traced them - and found that all the calls were coming from a house owned by a candomble priest ...

Oo-er. The spooky candomble cult represents the dark, illogical, voodoo side of modern Rio. It had its origins in African slave ritual; its adherents tend to come from the north-eastern Bahia province, and it offers an Eden of manumitted and trouble-free bliss to the enslaved and the wretchedly poor, provided they do what they are told by the priest. An unguessed- at world of blood- sacrifice, weird domestic rituals and grinning Jungian archetypes lies just beyond the smiling faces of the Bahian lady street vendors in their white lace threads, selling coconut cakes by the roadside.

One day I found myself in a museo di negro, gazing at a candomble icon - the figure of "Anastasia", a black girl slave, instantly recognisable by the muzzle that's cruelly clamped over her face in all depictions of her. Her image first appeared 20-odd years ago, as the picture of an anonymous slave in a travel book, demonstrating the correct use of "the Mask of Flanders", as the muzzle was called; torn from her context, she became, overnight, a cult figure and symbol of black repression. Anastasia shrines crop up everywhere, accompanied by little offerings of candles and cigarettes from the anonymous faithful - the latter, apparently in order to ask the gods to help a member of the donor's family to get off drugs.

I'm a sucker for odd belief systems so, outside the museum, I had my fortune read by a candomble priestess, a huge woman with hennaed hair, ham-bone forearms and a stack of lucky charms and divination aids laid out before her on a table: a pack of tarot cards, bank notes, cowrie shells, fridge magnets, cloves of garlic, bead necklaces and a crucifix on top of a gift-shop vase.

We got down to it. I cut the tarot pack three times and she examined the result. She assured me that I had financial worries (nothing new there, then), that I must soon make a great voyage (that'll be the flight back to London, I expect), and that good things would happen with my children, provided I was careful with my Gypsy Spirit. Well, I'll certainly try, I promised, but ...

"Bad things await you at your place of work," she said. "Big eyes are looking at you." If you mean the features editor, I said, I thought our relations were perfectly cordial, although ... She looked very serious. "You have powerful enemies," she said. "You are being helped by a divinity called Oxumare, like a serpent or a rainbow, but you must be strong yourself. You must make a sacrifice."

Mm-hmm, I said. And how do I do that? "You must bring a kilo of sweet potatoes on a white plate," she said, "and a bottle of honey and half a kilo of salad beans and come back here for sacrifice. You pay me 27 Reals [about pounds 9] more and I do it for you."

I confess I was rather disappointed. I had expected a spiritual journey. I'd have settled for a physical quest, like Hercules and the Augean stable. The last thing I expected was to be given a grocery list for the voodoo Sainsbury's.