HITTING THE HIGH NOTES Sally Bradshaw, opera singer, on the strange world of the castrati

The recent surge of enthusiasm for baroque opera has led to a plethora of productions as close as possible to the sound and atmosphere of the 18th-century originals. I have performed in theatres where the stage machinery is operated by hand and creaks loudly, and where the stage is lit by hundreds of candles - to the peril of singers in costumes with wide sleeves if they are tempted to gesticulate.

But one crucial aspect of baroque opera defies recreation, for reasons of both legality and barbarism: the mysterious singing voice of the castrato. In 18th-century Italian villages, crafty parents would sell their eight- year-old sons to church choirs or operatic schools willing to pay large sums for a voice with potential. These boys would undergo an operation which involved either the removal of the testicles or the severing of the connecting passage. As a result, puberty was prevented and a strange creature evolved - very tall, and inclined to be rounded.

"They usually become large and fat like capons," the French traveller Charles De Brosses wrote home from Italy in 1739, "with thighs, buttocks, arms, breast and neck round and plump like women. When one meets them in a crowd, one is astonished when they speak to hear such a tiny child's voice from these colossi."

The sexual ambiguity of these men, who were sometimes mistaken for women when they performed on stage, is partly responsible for their strange allure. In 18th-century operas, they wore camp costumes with extravagantly plumed helmets and little tutu-type skirts called tonnelets when creating heroic masculine roles, and they dressed in drag to act female characters.

But if we will never hear the singing of a castrato, these splendidly equivocal creatures remain fascinating. A film based on the life of the greatest castrato of all, Farinelli, has been taking France and Belgium by storm, and is due to open in Britain this summer.

Farinelli had, among other things, extraordinarily powerful lungs. He was able to hold a note for so long that people thought he was cheating - that he must have had an instrumentalist hidden somewhere to cover for him while he snatched breaths.

For the new film, the laboratory at Ircam in Paris has synthesised the voices of a male alto and a coloratura soprano together to create an eerie, flexible, other-worldly sound. One of the high points of the soundtrack is a single note held for a minute, which made my hair stand on end as I wondered if this was really what a castrato would have sounded like.

I doubt, though, that the synthesisers have quite done justice to Farinelli. Here's the impression of the 18th-century musicologist Charles Burney: "Though during the time of his singing he was as motionless as a statue, his voice was so active that no intervals were too close, too wide or too rapid for his execution. It seems as if the composers of these times were unable to invent passages sufficiently difficult to display his powers... No vocal performer of the present century has been gifted with a voice of such uncommon power, sweetness, extent and agility as Carlo Broschi detto Farinello." Farinelli's impact must have been rather like Joan Sutherland's, say, in her first Lucia at Covent Garden. Like her, he was lauded and courted as a star.

However, the private life of a castrato was not so happy; flattered and worshipped on the stage, he was very often ridiculed as a "demivir" (half man) off it. These men endured an extreme version of the double standard that was applied to all performers at the time: they couldn't be buried in consecrated ground because their stage appearances were sinful, yet they were treated as gods on stage.

Whether for hormonal reasons or simply as a reaction to the mutilation, castrati were apparently very prone to tears. Farinelli was no exception, and frequently succumbed to moods of weepy melancholy. But castration did not turn them into eunuchs: these stars were much in demand from married ladies. "They were forward with women who, as the scandalous gossips pretend, seek them out for their talents, which are endless..."

One castrato applied to Pope Innocent XI for permission to be married, on the grounds that the operation had been badly performed. To this the pope wrote in the margin "Che si castri meglio" (let the job be done properly), a heartless jest which gives us an idea of how hard it must have been for castrati to live happily at a time when any oddity was ridiculed.

It has been suggested that one day a singer will emerge from a village in the deepest part of some country known for brutality and poverty, and that he will be a real castrato. The music-loving public, naturally, would be both fascinated and horrified if this happened. The mere thought of such an operation arouses deep anxiety: enough for Freud to have created an entire psychoanalytic edifice around it.

But if it happened, I'm afraid people would flock to hear and to buy, just as they did two hundred years ago.