Tim Fountain: Sex Addict, Theatre Upstairs, Royal Court, London

The show ends somewhat later each night for Tim Fountain than it does for the people who've watched it. At the close of Tim Fountain: Sex Addict, he cycles off to have meaningless sex with the lucky person chosen for him by the audience from a group of applicants who have chatted to him live on his website.

The show ends somewhat later each night for Tim Fountain than it does for the people who've watched it. At the close of Tim Fountain: Sex Addict, he cycles off to have meaningless sex with the lucky person chosen for him by the audience from a group of applicants who have chatted to him live on his website. He takes a camera with him to film the post-coital verdict on his prowess, and that becomes the start of the next night's show.

The piece caused the obligatory paroxysm of controversy in the press at last year's Edinburgh Festival. In the course of the run, Fountain was forced to alter the format. The candidates for his charms were originally unsuspecting cruisers on a commercial internet sex site. They now know what they are letting themselves in for. For this transfer to the Royal Court's Theatre Upstairs, Fountain, the man who claims to have shagged more than 5,000 men (and one lesbian comic), has added a refinement (if that's the right word). Members of the audience can now nominate themselves (or be nominated) for the privilege of his evidently sizeable favours.

It's a very bizarre experience. Fountain comes across as a highly engaging, trustworthy Northern bloke, almost like an old-style children's TV presenter with an added sense of humour. As he gives you a graphic visual tour of what's available on the web (sites that cater for everything from "scat queens", to folk with a fetish for nookie while wearing Parkas), it feels less like a sordid trawl than a mark of Fountain's generous openness to the infinite absurdity of the human sex-impulse.

There are things that make me uneasy about the piece, though. The show is scrupulous about not giving away the addresses or phone numbers of the candidates. But I still came away with the sense that these people were being exploited.

There was a comic and telling incident when Fountain was trying to persuade one guy to ring him up in private and pass on his details. Fountain's assistant typed the message: "We promise we won't tell." That communal "we" was a bit of giveaway, not lost on the amused, voyeuristic audience. The message was swiftly amended to: "Oops, I mean I." A spectator might be tempted to make the moral argument that even exhibitionists deserve not to have such an exhibition made of them.

Then there's the safety issue: there are sex-murderers out there. Might we not be sending Fountain off to what could turn out to be a literal show-stopper? My anxieties were partly allayed when a female candidate declared that there was a rumour on the web that sex with Fountain never lasted very long. Presumably he has get it over with as quickly as possible each night so that he can put the theatre management's mind at rest that he's not ended up with a weirdo.

The oddity of the occasion is intensified by the venue. England's national theatre of new writing is not the first place you'd expect to find a weird variant of reality television.

To 29 January (020-7565 5000)