Reproductive maturation and attainment of maximal gonadal size in collared lemmings (Dicrostonyx groenlandicus) is facilitated by transfer from a short photoperiod (8 h light:16 h dark) to a long photoperiod (22 h light:2 h dark). However, reproductive maturation in lemmings born in 22 h light:2 h dark is impeded by exposure to the natal photoperiod of 22 h light:2 h dark. Data from adult lemmings suggest that prolonged exposure to a long photoperiod of 20 h light:4 h dark results in gonadal regression. The present experiments were conducted to test the hypothesis that adult lemmings exposed to a prolonged long photoperiod undergo testicular regression. Male collared lemmings were transferred at weaning (19 days of age) from the natal photoperiod of 8 h light:16 h dark to one of two long photoperiods: 22 h light:2 h dark or 24 h light:0 h dark (constant light). Gonadal mass was maximal in both groups 10 weeks after weaning. However, 20 weeks after weaning, gonadal mass in both groups was low (relative to that at 10 weeks), suggesting that regression had occurred. When testes mass was maximal 10 weeks after weaning, animals were transferred in groups from 22 h light:2 h dark to one of several shorter photoperiods: 20 h light:4 h dark, 18 h light:6 h dark, 16 h light:8 h dark, or 8 h light:16 h dark. Testes mass 20 weeks after weaning was greatest, that is, not different from maximal, in animals from the 20 h light:4 h dark and 18 h light:6 h dark groups, intermediate in animals from the 16 h light:8 h dark group, and low in animals from the 8 h light:16 h dark group. These observations indicate that transfer from 22 h light:2 h dark to a slightly shorter photoperiod permits maintenance of gonadal size, but transfer to a short photoperiod causes testicular regression. Body weight, bifid claw width and pelage colour did not change during spontaneous gonadal regression in animals housed in a long photoperiod for 20 weeks. These results suggest that prolonged exposure to an unchanging long photoperiod causes spontaneous testicular regression, and that photoperiod regulates reproductive and somatic parameters independently.