The acute and chronic effects of propylthiouracil (PTU) on serum thyroid hormone concentrations were assessed with that of methimazole as control. For the study of acute effects, among 54 untreated patients with hyperthyroidism selected at random, a single dose of PTU 200 mg and methimazole 20 mg were given to 29 and 25 patients respectively. And then 15 patients of each group were randomly selected for the study of chronic effects;PTU was given orally 100 mg t.i.d., and methimazole 10 mg t. i. d. for 4 weeks. Serum T₃, T₄ and rT₃ were assayed. The results suggested that PTU and methimazole might exert different effects on T3 production. It seemed that PTU also inhibited the conversion of T₄ to T₃ by reducing the activity of 5'-deiodinase, thus resulting in normalization of T₃. PTU is better than methimazole in treating severe hyperthyroidism and thyroid crisis as well. There was no significant difference between the chronic effects of both antithyroid drugs on serum thyroid hormone levels.