The present expts. were undertaken in order to determine the effect of direct hypophysial lesions upon the mating activities of [male] guinea pigs. Lesions of various sizes were placed directly in the hypophysis of 5 spayed [female][female]. Following: the operation estrous behavior was induced in all animals by inj. of ovarian hormones. Similar hypophysial lesions failed to abolish mating activities in 10 of 14 unspayed [female][female]. Nine of the 14 animals became pregnant, while the 10th, an animal with an endocrine disturbance, showed estrous behavior following inj. of ovarian hormones. 9 of the 14 unspayed [female][female] with hypophysial lesions ran regular ovarian cycles following operation; 2 ceased running cycles, that is, the vaginal membrane remained constantly closed. The 3 remaining animals ran cycles in which the period of opening of the vaginal membrane was greatly prolonged. The hypophysial dysfunction in these animals was not related to the extent of the damage to the hypophysis. The absence of mating behavior following hypothalamic lesions is evidently due to the destruction of some mechanism within the hypothalamus which is essential for this behavior and not to a disturbance of hypophysial function.