It is well known that some volatile anaesthetic drugs, such as halothane and isoflurane, alter the functions of the human thyroid gland, but the action of other anaesthetic drugs, such as thiopental, midazolam and ketamine, on thyroid function is still unknown. We have investigated the effects of these three drugs on the functional properties of human thyrocytes cultured in monolayers or follicles and stimulated by TSH. Thiopental, midazolam and ketamine induced total suppression or a partial reduction, depending on the dose administered, of cyclic AMP (cAMP), follicular thyroglobulin (Tg) and free tri-iodothyronine (FT3) production. In contrast, free thyroxine levels increased in the medium of thyrocytes cultured as follicles. Small doses of the drugs did not affect thyrocyte production. The inhibiting effect of thiopental, midazolam and ketamine on Tg and FT3 production seems to result from the inhibition of cAMP production and 5′-deiodinase.