There are two iodothyronine 5'-deiodinases in the rat anterior pituitary gland. The type II 5'-deiodinase is responsible for the observable T4 to T3 conversion in pituitary tissue, both in vivo and in vitro. This type II enzyme has kinetic characteristics which distinguish it from type I activity, and it is also insensitive to inhibition by propylthiouracil. Iopanoic acid is both a substrate for, and an inhibitor of, the pituitary type II 5'-deiodinase. Studies in the rat and in man using iopanoic acid as a probe indicate that conversion of T4 to T3 within the anterior pituitary is a necessary step in the expression of thyroid hormone effects on the thyrotrophic and somatotrophic cells after T4 administration. Amiodarone, which has effects on the human pituitary-thyroid axis similar to those of iopanoic acid, has little inhibitory activity on T4 5'-deiodination in rat pituitary homogenate. Phenytoin, which lowers the serum T4 after chronic administration without inducing an increase in thyrotropin, could theoretically act by enhancing intrapituitary T3 production. In actuality, phenytoin shows only inhibitory activity in vitro on type II 5'-deiodination in rat pituitary and brain homogenates and on type I activity in rat liver homogenates. Phenytoin treatment in vivo has no significant effect on T4 5'-deiodination in pituitary or liver homogenates either. Evidence is discussed regarding whether the increase in type II 5'-deiodinating activity in hypothyroidism, and the decrease in hyperthyroidism, are beneficial defense mechanisms; this is likely to be so in some parts of the body, but may ot be in the thyrotrophic cells of the anterior pituitary.