Male deer mice (Peromyscus maniculatus bairdii) were exposed to a long (15L:9D) or short (6L:18D) photoperiod during four stages of development. At 6 weeks of postnatal age degree of pubertal development was determined by measurement of testis and seminal vesicle weights and area of the androgen-dependent sebaceous gland. Continuous exposure to a short photoperiod beginning before conception (i.e. mother kept in short days), at conception, or at birth markedly retarded pubertal development. Initiation of short-day treatment after weaning at 3 weeks of age was partly effective in inhibiting puberty. Males exposed to long days beginning at weaning were stimulated fully, however, having reproductive organs as large at 6 weeks of age as those of males housed on long days for the entire experiment. These results indicate that pubertal development in male deer mice is regulated by early postnatal, but not prenatal, photoperiod. However, early after weaning, males are more responsive to increasing than to decreasing photoperiod.