Advertisements get everywhere: on eggshells, down golf holes and soon, we hear, in space. So it is no surprise to find that they have made their way on to computer screens.

Subscribers to Prodigy, one of America's on-line information services, are now shown advertisements running along the bottom of the screen. But - and this is where it gets clever - some of these are 'interactive'. Click on the Toyota ad, produced by Saatchi & Saatchi in New York, and you will find yourself drawn into a strange new world. You will be asked if you want full details of any Toyota model, if you want to draw up a service schedule for your own car, or if you want to cross-examine a Toyota engineer. Most popular, according to Saatchi, is the 'chat line' which allows you to exchange views with other Toyota freaks. It is not really chatting, of course: everything has to be written down.

Although Prodigy is available only in the US, anyone in Britain with a CD drive can sample interactivity. What Car? magazine and Saab have produced a CD-rom that cuts together a BBC2 test of the car with technical information.

Users can interrogate the disc for more information, and can even 'paint' it by selecting a colour from a palette.

If all this seems like a bizarre way of spending your time, the advertising industry disagrees. Erica Gruen of Saatchi & Saatchi in New York says that computers 'have the promise to be a medium in their own right. In five to 10 years' time, 50 per cent of US homes will have personal computers, and 20 per cent will be connected to on-line services'. But she also says that on-line advertising is a useful testbed for a potentially vast new industry -interactive commercials on television.

There is nothing new about interactive advertising: all it means is that the process is two-way. Every time a newspaper reader fills out a coupon, he or she is 'interacting'; so is a potential buyer talking to a salesman. He asks questions, the salesman answers them and the conversation moves on to more specific points. The more specific the questions are, the closer the salesman is to getting a sale.

A standard television commercial, by contrast, is a one-way lob.

'Advertising tends to talk at people - it's a monologue,' says John Crowley, director of media communications at the agency Chiat/ Day. 'Interactivity turns it into a dialogue: it's like having the man from the Pru in your living room.'

So far, the only way a commercial can be interactive is by showing a phone number and hoping people ring it. Signals can go into a television, but not back from it. Come the information superhighway, though, and they will fly backwards and forwards. When homes are connected to fibre-optic 'broadband' networks, and televisions have built-in computers, viewers will be able to send messages back to do their home shopping, order their video - or chat with Toyota owners.

One day the traditional 30-second slot will - the agencies hope - be replaced by slots that are merely the outer wrapping for goodies within. By zapping the remote control viewers will engage in elaborate conversations with, for example, a holiday company. They will be asked whether they are looking for relaxation or adventure, what sort of accommodation they want, how hot they like their weather, and when they want to go. They will then be shown a film promoting a particular package or hotel which they can then book by hitting the handset again.

Andrew Curry, interactive television manager of Videotron, which has the cable franchises for large chunks of south and west London, says he thinks this sort of commercial could be with us in six or seven years. But Videotron subscribers will be able to take part in an experiment in interactive advertising this winter.

Working with Chiat/Day, Videotron is about to launch a month- long trial scheme. It will use a form of interactivity that is not two-way but does allow viewers to feel they are talking back to the broadcaster. By pressing one of four buttons on the handset, they can watch a football match from a different camera angle, or perhaps watch a replay: they are in fact selecting different channels, but that is not how it seems.

In Canada, Videotron has carried advertisements for Ford in which viewers are invited to select the type of vehicle they are interested in: small car, four-wheel drive, and so on. They then press a button and watch one of four ads.

The London trial will use the same technology, but rather than making just one choice at the beginning of the slot, viewers will respond to series of prompts, moving down a branch line towards a particular product. The aim, Mr Curry says, 'is on the one hand to see what creative people will do with this sort of technology, and on the other hand to see how viewers use it'.

Despite the encouraging results from Prodigy, no one knows whether viewers will take any more notice of interactive advertisements than of ordinary ones. Agencies are staring hard at their navels to work out how to make viewers press the button that takes them into the commercial. The skills they need will be similar to those used by Reader's Digest in its attempts to persuade you to open a junk mail envelope, and they will be honed first on the on-line services. Stand by for a once-in-a-lifetime unmissable offer, coming soon to a computer screen near you.

An expanded version of this article appears in the newsletter 'Interactive Entertainment'. Tel 071-240 6646.

(Photograph omitted)