In this study, we investigated the sex hormone regulation of 5'-iodothyronine deiodinase activity, which is responsible for enzymatic conversion of thyroxine into the bioactive form, triiodothyronine. Pituitary homogenates and liver microsomes from: 1) ovariectomized rats injected with 17-beta-estradiol benzoate and/or progesterone (0.7 and 250 microg/100 g body weight, respectively, subcutaneously, over 10 days); 2) male castrated rats treated or not with 0.4 mg/100 g body weight testosterone propionate, intramuscular, over 7 days, were assayed for type 1 and type 2 deiodinase activity in the pituitary. Enzyme activities were measured by release of (125)I from deiodination of (125)I reverse triiodothyronine under varying assay conditions. Estrogen stimulated anterior pituitary and liver type 1 deiodinase activity in ovariectomized rats (45 and 30 %, p < 0.05). Progesterone inhibited the liver enzyme (40 %, p < 0.05), and had no effect on the pituitary, but in both tissues, blocked estrogen stimulatory effect on type 1 deiodinase. In males, testosterone normalized the reduced liver type 1 deiodinase of castrated rats. However, in the pituitary, castration increased (50 %) type 1 deiodinase independent of testosterone treatment, suggesting the existence of a inhibitory testicular regulator of pituitary type 1 enzyme. Treatments did not alter pituitary type 2 deiodinase activity. In conclusion, gonads and sex steroids differentially modulate type 1 deiodinase activity in rat pituitary and liver.