Five experiments consistently demonstrated the existence of a factor(s) in the urine of female mice which stimulates gonadotropin secretion in male mice. The factor, presumably a priming pheromone, is not found in the urine of intact or castrated males nor is it in the urine of female hamsters. Furthermore, the potency of female mouse urine is apparently independent of ovarian state. In the response of the male, the release of LH is more dramatic than that of Fsh. as little as 10 min of exposure to female urine produced significantly elevated LH levels at 30 min. Finally, the male's responsiveness is largely, and possibly entirely, independent of his sexual experience. These results are discussed within the framework of a two-way pheromonal system by way of which urinary factors of both sexes modulate gonadotropin and gonadal hormone secretion in the opposite sex.