The effects of testosterone on the mesonephric collecting ducts ofTriturus italicushave been studied. During March and April these ducts hypertrophy naturally and secrete, but during the rest of the year their epithelium is regressed. Castration was followed by involution of the ducts so that the regression persisted. Testosterone given in massive doses to intact and gonadectomized animals in the regressed phase resulted in full secretion.