Female deer mice were exposed to a short day (SD, 6L:18D) photoperiod beginning during 1 of 3 stages of life. In the first experiment, exposure to SD during adulthood resulted in a minimal disruption of reproductive condition; many females bore 2 litters after the onset of this treatment. In the second experiment, females reared on SD from weaning matured normally, as measured by vaginal introitus; however, vaginal closure occurred in approximately one-half of these females by 9 weeks of age. In the third experiment, females were born of mothers housed on either an SD or a long day photoperiod, and were continued on the maternal photoperiod until 6 weeks of postnatal age. The SD photoperiod markedly inhibited reproductive maturation as measured by vaginal patency, ovarian weight, and uterine weight. A comparison of reproductive organ weights and vaginal condition provided evidence for the validity of the latter measure as an index of reproductive state. As assayed by the present testing procedure, the sensitivity of the reproductive system to photoperiod decreases as a function of age in female deer mice.