Female meadow vole weanlings exposed to odours from homogenates of bladder and testis taken from sexually mature males showed more rapid maturation (earlier vaginal perforation, heavier uteri, adrenal glands, and ovaries with more and larger Graafian follicles, and cornified smears lacking mucus) than controls exposed to distilled water. Females exposed to kidney homogenates showed similar responses except that their time of vaginal perforation and adrenal weight did not differ from controls, and some had mucous smears. Seminal vesicle, preputial, and prostate homogenates produced only occasional effects on these indicators of reproductive maturation. The pheromone responsible for these effects, known to be present in urine, is deduced to enter the urine stream at the kidney rather than at the level of the sexual accessories.