Male deer mice (Peromyscus maniculatus), born of mothers housed on a longday (LD) photoperiod (15:9 hr light/dark), were either switched to a short-day (SD) photoperiod (6:18 hr) at birth or continued on their prenatal LD photoperiod. From weaning until 6 wk of age, the males were housed either in cohabitation with an adult female or in social isolation. Males reared on an SD photoperiod had smaller testes, seminal vesicles, and ventral sebaceous glands than did males reared on LD. Postweaning exposure of SD males to females stimulated reproductive organ growth as measured at 6 wk of age. Both photic and social stimuli regulate reproductive development in male deer mice. Positive social cues can stimulate maturation even in the presence of negative photic cues.