Hypothyroidism induced by thiouracil has no effect upon the reproductive system of the adult [male] rat as judged by the ability to sire litters. Prolonged thiouracil admn., associated with hypothyroidism, in the adult [female] rat, does not cause sterility but does interfere with continuation of gestation, causing resorp-tion of embryos in 100% of the cases. If the drug is given over a period of less than 100 days some rats do deliver litters which are normal in growth and development and reproduce normally.