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Marmoset monkeys were treated with oral doses of 0.1, 1 or 3 mg 3,4,3',4'-tetrachlorobiphenyl (TCB) per kg body weight 2 times a week for 18-23 weeks. Histological examination of the thyroid gland revealed a dose-dependent follicular cell hyperplasia. The morphological changes were associated with various disturbances of thyroid function. The average serum thyroxine (T4) levels during the treatment period were reduced by more than 99% in monkeys receiving 3 mg TCB/kg, by 81% in marmosets on a dose of 1 mg TCB/kg, and by 35% with 0.1 mg TCB/kg. The reduction in serum T4 levels was established from the earliest time point (2 weeks) throughout the whole dosing period (18-23 weeks). The reduction in serum T4 levels was reflected in decreased free thyroxine (FT4) index in the 1 and 3 mg TCB/kg dose groups. Serum triiodothyronine (T3) levels were lowered in the 3 mg/kg dose group already after 2 weeks. Evidence for decreased binding to carrier proteins is suggested by increased T3 resin uptake in the highest dose group. Levels of thyrotropin (TSH) were increased in the highest dose group as a feedback response to the dramatically reduced serum T4 levels.