Four experiments examined the role of photoperiod in the regulation of seasonal breeding in the prairie vole. Adult male voles maintained in short (8L:16D) as compared to long (16L:8D) photoperiods for 10 wk had reduced testicular and seminal vesicle weights, but fertility was not impaired. Male prairie voles reared from birth until 35 days of age in short as compared to long photoperiods also had reduced testicular and seminal vesicle weights, as well as diminished fertility. The incidence of pregnancy did not differ between long- and short-day female voles paired for 6 days with long- or short-day males (93%, 86%, 89%, and 88%, respectively). Photoperiod did not affect the incidence or the timing of postpartum pregnancies in long- or short-day females paired with long-day males through the birth of several litters. Adult male prairie voles retain only marginal sensitivity to short photoperiods, maturing males are highly responsive to short days, and adult females are insensitive to photoperiod. These data suggest that termination of the breeding season in the autumn may be due to the lack of fecund males in the population.