Macroscopically the largest mammary glands were observed in the group that had been given chlorpromazine and estrogen simultaneously. Reserpine caused an increase in the development of the ducts and alveoli. Secretory activity could be observed in the alveoli. The simultaneous administration of estrogen tended to suppress the changes produced by reserpine. When reserpine and chlorpromazine were given simultaneously, the effect of chlorpromazine seemed to dominate with the doses used. Chlorpromazine caused multiplication and epithelial proliferation of the ducts. In the chlorpromazine series, alveolar formation was less than in the reserpine series and hardly any signs of secretory activity in the alveoli were observed. The simultaneous administration of estrogen produced no changes in the effects of chlorpromazine.