Cis-diamminedichloroplatinum (II) (cisplatin) is a widely used anticancer drug which induces many sideeffects, but its action on the thyroid gland is still unknown. We have investigated the effects of this drug on human thyrocytes cultured in monolayers or in follicles and stimulated with 200 μU TSH/ml. After 72h in culture, different concentrations of cisplatin (15, 30 and 75 μm) caused partial or total inhibition of cyclic AMP (cAMP), thyroglobulin (Tg) and tri-iodothyronine (T3) production, whereas thyroxine levels increased in the medium of thyrocytes cultured as follicles. Small doses of the drug did not affect thyrocyte production. Decreases in neutral-red uptake by thyroid cells and in intracellular lactate dehydrogenase, α-hydroxybutyryldehydrogenase and creatine phosphokinase activities were induced by 30 and 75 μm cisplatin. These data show that high concentrations of cisplatin had a cytotoxic effect on thyrocytes. Cisplatin also induced inhibition of the production of cAMP, Tg and T3.