The metabolism of testosterone by liver microsomes was investigated in male and female rats under altered states of the thyroid. The incubation of testosterone with liver microsomes produced mainly hydroxylated polar metabolites innormal male rats and Δ 4-reduced metabolites in female rats. The treatment with thyroxine markedly decreased the hydroxylation of testosterone by liver microsomes in male rats, but caused no significant alteration in female rats. Thyroidectomy also markedly decreased the formation of hydroxylated metabolites of testosterone in male rats, but caused no significant alteration in female rats. The content of cytochrome P450 in the liver microsomes, the magnitude fo testosterone-induced spectral change and the binding capacity of the cytochrome with testosterone were decreased in thyroxine-treated male rats, but was not altered significantly ni the thyroidectomized rats except for a slight decrease in the binding capacity of cytochrome P450. On the basis of these results, the mechanisms of the decrased in the hydroxylating activity of testosterone in the thyroxine-treated and thyroidectomized male rat is discussed.