Tamerlano, Theatre Royal Glasgow <!-- none onestar twostar threestar fourstar fivestar -->

Charles Burney, an 18th-century traveller, tells us that he saw a sign over a shop in Naples: "Boys castrated here." It's a tall story, for castration was illegal then, as now. But it was a thriving industry, and it produced some of the most amazing opera singers.

Of course, they have now all disappeared. If you want to perform an old opera you have to give their parts to women or male altos. The trouble is that these artists do not remotely sound like castrati, and this was the undoing of Scottish Opera's production of Handel's Tamerlano.

It was disappointing, for this performance was set up by the same team that created the superlative version of Semele last year, John la Bouchardière (director) and Christian Curnyn (conductor). Curnyn, as before, made wonderful music with the Scottish Opera orchestra: vital, eager, fluent.

And the producer showed some intelligence: the opera was sung in period dress in the first half, modern clothes in the second, stressing a link between the brutality of medieval tyrants and that of modern terrorists. The scene looked like the interior of the British Museum, with metopes on the wall and large sculptures dominating (in the third act it had been wrecked and despoiled).

It could have worked perfectly well. But Tamerlano is not a semi-oratorio like Semele. It is a full-blown opera seria, consisting mostly of arias. This was where it came unstuck. The parts of Tamerlano and Andronicus were sung in 1724 by great castrati.

The Glasgow singers of these parts were Max Emanuel Cencic and William Purefoy, both countertenors. Cencic had a secure control of his florid part, but Purefoy lacked fire, as well as being a stiff stage presence. Above all, the countertenor voice, however good, is soft-edged and husky, whereas the castrati sounded brilliant, metallic, like a trumpet.

This upset the balance. You looked forward to the numbers sung by Gail Pearson as Asteria. She sounded pure and gleaming, even at times a little coquettish. The other female singer, Jennifer Johnston, was allowed to shape the part of Irene as a comic role, which slightly spoilt the accuracy of her delivery, especially when she was required to climb over the scenery.

The role of Bajazet, father of Asteria, was mercifully given by Handel to a physically intact tenor, and here Tom Randle was a fine old barnstormer of an actor. But he sang only approximately, with unsteady approach to phrases and disconcerting shouts in the high parts. The smaller part of Leon (the bass Jonathan Best) was sonorous and unobtrusive - aptly, since he was dressed as a butler.

But arias rolled past in baroque profusion, and many of them were designed for castrati. You started to dread yet another duff effort from what should have been the two superstars. It made the evening seem interminably long. Perhaps we should try to find that shop that Burney mentioned.

To 18 November (08700 606 647)