In a series of five experiments, young male deer mice (Peromyscus maniculatus bairdii) were exposed to photoperiods ranging in length from 3 to 19 h per 24-h period, as well as to constant light and constant darkness. Reproductive organ growth as measured at 6 weeks of age was inhibited by less than or equal to 11.5 h of light. In males receiving 11.5-19 h of light, there was a quantitative increase in reproductive organ size as a function of photoperiod. Rather than a 'critical' photoperiod partitioning reproductive inhibition and stimulation, there was a zone of gradual transition from inhibition to full stimulation. Histological analysis of testes demonstrated that spermatogenesis was stimulated by long photoperiods. Examination of the epidermal surface of the penis by scanning electron microscopy indicated that penile spine development was also regulated by photoperiod.