Since recent in vivo evidence suggests that the benzofuran antiarrhythmic drug amiodarone has a direct toxic effect on the human thyroid gland, we have investigated the effects of both amiodarone and its metabolite desethylamiodarone on a novel immortalized functional human thyrocyte line (SGHTL-34 cells). Desethylamiodarone markedly reduced cell number as assessed from both DNA and protein content. Few cells were left after 24 hr exposure to 12.5 micrograms/ml; the concentration producing death of 50% of cells (EC50) was 6.8 +/- 1.1 micrograms/ml (mean +/- SE, N = 15). Amiodarone was much less potent, producing a maximum decrease in cell number of approximately 25% at concentrations up to 50 micrograms/ml. The effect of desethylamiodarone was seen within 24 hr of culture. T3 in concentrations up to 0.75 micrograms/ml had no effect on the action of amiodarone or desethylamiodarone on SGHTL-34 cells. Light microscopy demonstrated vacuolation of SGHTL-34 cells after 4-day culture with either the drug or its metabolite. Studies using primary cultures of human retroorbital fibroblasts demonstrated that the greater cytotoxicity of desethylamiodarone was not confined to thyrocytes. When SGHTL-34 cells were incubated with 2.5 micrograms/ml desethylamiodarone for 4 days, 71.7 +/- 0.9% was taken up by the cells; there was no detectable conversion to amiodarone. Incubation of thyrocytes with 50 micrograms/ml amiodarone for 4 days resulted in the uptake of 80.1 +/- 2.1% by the cells. In addition, 5.0 +/- 0.1% of the amiodarone was converted to material with the same retention time as desethylamiodarone standard; of this material, 72.9 +/- 2.8% was taken up by the cells. We conclude that desethylamiodarone, at concentrations near those found in the plasma of patients on long-term amiodarone therapy, exerts a direct cytotoxic effect on human thyroid cells in short-term culture. This effect may play a role in the aetiology of clinical thyroid disease during amiodarone therapy. We suggest that, since the effect is not restricted to thyrocytes, desethylamiodarone may play a role in the aetiology of amiodarone toxicity which occurs clinically in many tissues.