Studies were made of the steroid secretion pattern of gonadectomy-induced adrenal cortical tumours in mice. Morphological and chemical data were collected from intact and gonadectomized female mice (F1 hybrids of strain DBA/2WyDi female × strain CE/WyDi male and the reciprocal cross). Morphological data – In spayed animals with adrenal tumours, the weights of the hypophysis, kidney and submaxillary gland were significantly greater than in intact controls (without adrenal tumours). Body and thymus weights were similar in the spayed and control mice, and there was no femoral osteoporosis in either group. Histologic changes in kidneys of tumour mice were compatible with androgenic stimulation. Chemical data – Incubation of normal and neoplastic adrenal tissue with progesterone 4-14C showed conversion to corticosterone and deoxycorticosterone. The conversion was accomplished more efficiently by normal than by neoplastic adrenal tissue. There was no evidence for in vitro conversion of progesterone to oestrogens or androgens nor for the storage of these compounds in the adrenal tumours. Corticotrophin content in the tumour mice increased in proportion to the increased weight of the pituitary gland, the concentration of corticotrophin being the same as in the controls. The morphologic and chemical data taken together suggest an enhanced androgen secretion by the adrenal tumours, but no increase, per unit weight of tissue, in corticosteroid (C21 steroid) production.