Specific derangements of thyroid and gonadal functions were observed in athymic nude and neonatally thymectomized mice. Such endocrine alterations are already established during the perinatal period and maintained through adult life. Passive transfer of lymphoid cells from normal donors does not prevent such alterations. In nude mice thymus implantation at birth fully reconstitutes oestrogenic function, but thyroid and progestational functions remain defective. Peripheral endocrine glands (thyroid, adrenals and ovaries) respond normally to ACTH, TSH and LH. Thus the thymus may well have a basic role in the organization of the adult hypothalamus-pituitary axis for thyroid and sexual functions.