Male hamsters were raised on short (SD) or long (LD) photoperiods from birth, and the expression of sexual behavior at 6, 7, 10, and 14 weeks was compared. Neither mounting, intromissions, ejaculations, nor ultrasonic vocalizations differed in the two groups until 14 weeks of age, when these behaviors were performed more often by LD males. Sperm appeared in daily penile smears at an earlier age in SD than LD males, but LD males had larger flank glands and testes beginning at 6 and 7 weeks, respectively. Female siblings of the males in this study were mated around 7 weeks of age to adult males. Almost all females on both photoperiods conceived normally, and greater than 90% of the offspring delivered survived to weaning in both groups. Together, these results show that hamsters reared on SD are behaviorally and physiologically capable of reproducing for a period of time after puberty. The possibility that social influences or environmental factors other than photoperiod act on the juvenile hamster to retard reproductive development is discussed.