Onset of sexual maturation was determined in weanling male collared lemmings exposed to one of three experimental regimens of different photoperiods before and after weaning. Animals gestated in photoperiods of either 16h light:8h dark or 8h light:16h dark. Those from 16h light:8h dark were transferred at 19 days of age to either 20h light:4h dark or 8h light:16h dark; those gestated under 8h light:16h dark remained in that photoperiod throughout the experiment. After exposure for 15, 20, 25 or 30 days to the postweaning photoperiod, animals were killed and the following parameters assessed: body weight, testes weight, seminal vesicle weight, the presence or absence of epididymal spermatozoa and serum concentrations of prolactin, testosterone and corticosterone. All parameters except serum testosterone were significantly influenced by photoperiod. Animals housed under 8h light:16h dark had significantly greater body weights than those housed under 20h light:4h dark, a response that differs from that reported for other arvicoline rodents. The group gestated on 16h light:8h dark and transferred on day 19 to 8h light:16h dark had lower testes and seminal vesicle weights than the other two groups, and mature spermatozoa in the epididymides appeared 5 days later than in the 20h light:4h dark group. Serum prolactin was largely undetectable in animals from both 8h light:16h dark groups, but all males housed in 20h light:4h dark had 2.0-15.0ng prolactin ml-1. Concentration of serum corticosterone was higher in animals weaned into long photoperiod, and decreased with age. These data indicate that weanling male D. groenlandicus are reproductively photoresponsive, but use a decrease in photoperiod, not static short-photoperiod exposure, to alter the rate of development. Prolactin was largely undetectable in animals exposed to short photoperiod, indicating that high concentrations of this hormone are not important for maturation. Low prolactin concentrations in animals in short photoperiods may mediate the annual moult to white pelage. The short-photoperiod-mediated decrease in corticosterone may play a role in seasonal changes in body weight and composition.