In this study we show that 6-propyl-2-thiouracil (PTU) treatment of Wistar rats from birth up to day 26 p.p. retards the morphological differentiation of Sertoli cells, and prolongs the proliferation of these cells up to day 30. Sertoli cell numbers per testis, determined at day 36, were increased by 84% compared to controls. PTU treatment increased serum thyroid-stimulating hormone (TSH) levels and reduced serum levels of thyroxine (T4) from 5 days onwards, indicative of severe hypothyroidism. Follicle-stimulating hormone (FSH) levels were reduced from day 5 to 9, normal at day 12 and 16, and reduced again from day 20 to 36. Inhibin levels were decreased from day 9 to 20 and increased at 36 days of age. The increase in the number of Sertoli cells per testis in PTU treated rats, as has been reported in the present study, is likely to be responsible for the increased testis size observed by other groups (1) in these animals, when adult.