A sequence of 4 experiments examined the effects of prepubertal and adult males on the sexual maturation of young female house mice. The results support 3 conclusions: (1) the presence of a prepubertal male or of urine from prepubertal males does not affect the timing of sexual maturation in young female house mice; (2) the maturation-accelerating pheromone produced by adult males is present in the bladder urine of intact adult males but is absent from both excreted and bladder urine of castrated males; and (3) young females caged with 7 prepubertal males or with a castrated adult male mature earlier than control females caged alone. Results indicating that the presence of a castrated male leads to earlier sexual maturation of young female mice differ from previous findings. A possible explanation for this contradictory result is based on the ability of young weanling female mice to acclimatize the thermoregulate when separated from the dam and litter-mates. A model for density-feedback population regulation in house mice involving pheromones produced by males and females is presented.