Neil Bartlett addresses the main problem with his latest piece straightaway. None of you, he tells us rather sternly, knows the words to the religious rites of confirmation, baptism or marriage. He is standing at the front of a medical lecture theatre in the Royal London Hospital. Everyone, reduced by the setting to students, shifts nervously in their seats. I try to recall the various sacraments in their right order. Does Penance come before Ordination? What is Holy Eucharist anyway?

Bartlett's dramatic monologue, The Seven Sacraments of Nicolas Poussin, has as its focal point a series of religious paintings by the 17th- century artist, tracing the passage of the body from baptism to death. Each features a protean, yellow-robed protagonist who changes age and sex from one painting to the next. Helped by his friends - video artist Robin Whitmore and lighting designer Rick Fisher - Bartlett re-enacts these paintings, casting himself as the yellow-robed form.

The difficulty is that we are never allowed to see the paintings. Bartlett describes them meticulously and we are given projected details, but the effect of watching him balanced precariously on a medical lecture bench, draped in yellow, relating how in the corner of the frame an altar-boy is holding an unlit candle to the flame of another, is one of dissatisfaction. Too often, the curiosity he presumably means to incite tips over into frustration. "The first painting, as you can see," he says at one point and we all squint at a postcard reproduction taped to a blackboard. Considering those involved, the visuals are disappointing and, despite Bartlett's intriguingly Mephistophelian demeanour, at times I wondered if he was a charismatic enough performer to hold our attention for a full hour and a half.

It is, however, the interplay between script and site that carries the piece. How Bartlett persuaded the Royal London Hospital to house it I don't know. There is something weirdly sacrilegious about walking past waiting-rooms and wards on your way to the theatre. But Bartlett handles this tricky tension with a kind of grieving sensibility. References to life in the hospital are by far the most affecting: he compares the curtains kept drawn over Poussin's paintings to the curtains drawn round a patient's bed during an examination. The relating of how the sheet of glass which separates the morgue viewing-room from the body has to be cleaned often because it becomes covered with the frantic handprints of the bereaved has most of the audience in tears.

Like life itself, this piece isn't perfect. It's just that for one performer, grappling with the questions of mortality and the sacred and profane is a lot to take on single-handedly.

Royal London Hospital, E1 (0181 741 2133); today & Mon.