The hair on my neck crawls when I see Morris dancers performing, because their quaint costumes and tunes reach far back into our pagan past and raise apprehensions that defy analysis. What is the origin of their white shirts and trousers, the white handkerchiefs waved in their hands, the flowers in their straw hats and the bell-pads on their ankles? What is the significance of the hobby horse, worked by a man inside a dummy head? And what is the meaning of the fool, who runs round belabouring spectators with a blown-up pig's bladder and a lamb's tail?

Whitsun is the traditional peak of the Morris men's year, and this weekend teams will be out all over the country, especially on Monday. None will be more active than the Gloucestershire Morris Men, who are due to perform in six different places, including Broadway (at 10.30am) and Hidcote Manor (at 12.30pm and 1.45pm).

To learn what makes them tick, I joined them for supper one evening at the Black Horse in Cranham, a village tucked into a fold of the Cotswolds high above Cheltenham. Already the side had danced three times that day, and at 6pm they sat down in the pub amid the jingle of bells and roars of laughter to a supper of beef and Guinness pie.

My mentor was Steve Rowley, resplendent in a coat of tatters - hundreds of strips of coloured material, each (traditionally) torn from the petticoat of a conquest. Once the European representative of a computer firm, now a sculptor, Steve was refreshingly straightforward about his hobby. Suggestions that he is waking up the land from its winter sleep leave him cold. No, he says: Morris dancing is pure entertainment.

Certainly it has medieval origins, and a century ago almost every Gloucestershire village fielded a team (the name may be a corruption of "moorish", once a synonym for anything outlandish). But in the early 1900s the tradition nearly died out. Its survival owed much to the enthusiasm of Cecil Sharp, a professional musician and teacher who spent years collecting songs and dances. His work led to a revival in the 1930s, and now there are over 400 teams in Britain, besides others in such unlikely places as Australia and Bahrain.

Just as the grotesque horse (which can gnash its teeth, blink its eyes and shed tears) is still liable to frighten children, so the strangeness of the clothes increases mystique and creates the impression that Morris men are not quite human. But behind their antique facade they are reassuringly normal. According to Steve Rowley, "One reason we dance is to keep tradition going. But the main thing is that it gives us a kind of companionship we couldn't get elsewhere."

Even if its significance has been lost in the mists of time, tradition survives in many ways. The dances and tunes - Orange in Bloom, Constant Billy, Young Collins - are centuries old, and some have obvious echoes of fertility rites: in Bean Setting the men jab at the ground with short sticks, as if dibbing-in seed. The leader of each group is known as the Squire, the treasurer as the Bagman.

The Gloucestershire men train assiduously all winter. Then, come 1 May, they sally forth and dance until September, planning their programme to take in favourite hostelries. As in the old days, vigorous dancing is followed by vigorous drinking and singing: half the point of the exercise is to pile into the pub afterwards for a few pints and a rousing singsong. Last Saturday in the Black Horse, the atmosphere was highly convivial. Yet when the team began to perform in the road outside, I swear magic crept back into the air.

It was a damp, grey evening, spitting with rain; but as the dancers twirled against the grey limestone of the cottages, and the thin notes of the pipe and tabor went out over the valley, people began to filter up the steep village streets as if drawn by the Pied Piper, and time, far from standing still, took a rapid spin backwards to a simpler, less frantic age.