The northern fur seal (Callorhinus ursinus) is a migratory marine mammal belonging to the family Otariidae that breeds on the islands in the North Pacific Ocean, Bering Sea, Sea of Okhotsk, and near the coast of California during summer (Peterson 1968; Gentry 1998). Their reproduction is characterized by extreme polygyny and highly synchronized annual cycles that are concentrated around the boreal summer (Peterson 1968; Trites 1992; Gentry 1998). Females arrive at breeding sites in early July, give birth within 36 hours, and then enter estrus and copulate within a few days (Peterson 1968). Although active gestation persists for approximately eight months, the total duration of gestation period is approximately one year because of the embryonic diapause (delayed implantation), which accurately synchronizes the annual reproductive cycle (Craig 1964; Boyd 1991). Males return to the breeding sites approximately one month before females, establish their territories in the breeding area, and maintain breeding activities for approximately two months (Peterson 1968). Photoperiod is commonly cited as the main trigger for the synchronization of the annual reproductive cycles of pinnipeds (Boyd 1991; Atkinson 1997; Boyd et al. 1999), and this theory is supported by several studies. Temte (1985) compared the mean dates of parturition between the two wild populations of northern fur seals in Alaska (57°N) and California (33°N) and suggested that a mean photoperiod of 12.5 h/day in autumn acts as a trigger for the termination of embryonic diapause and initiation of implantation to synchronize parturition (also see Spotte and Adams 1981). Kiyota et al. (2009) also observed a significant latitudinal variation in the parturition of captive northern fur seals in Japan, which supported the theory of photoperiodic control of the reproductive cycle. The effect of photoperiod on reproductive cycles has been only experimentally demonstrated in harbour seals (Phoca vitulina) (Bigg and Fisher 1975). Bigg and Fisher (1975) demonstrated that estrus occurred earlier than usual during extended photoperiods and was delayed during shortened photoperiods. However, Daniel (1981) did not detect any effect of a shortened photoperiod on the rate of blastocyst development in northern fur seals. Boyd (1991) interpreted the finding of Daniel’s study to be the result of the relatively insignificant shortened photoperiod and/or the short study period. Seasonal changes that occurred in the testicular activity of male fur seals in captivity corresponded with those that occurred in males in the wild (Kohyama et al. 1999; Tsubota et al. 2001). These studies indicate that photoperiodic control of the male reproductive cycle is similar to that of female reproductive cycle, but this photoperiodic effect has not been extensively studied (Boyd 1991). Photoperiod has been reported to control the timing of implantation and parturition in pinnipeds (Temte 1985). However, this interpretation has not been applied to the timing of estrous and ovulation of non-pregnant females or the testicular activities of males. Other mammals, such as Siberian hamsters (Phodopus sungorus) and gray mouse lemurs (Microcebus murinus), are defined as long-day breeders because their reproductive activities increase in response to day length as the seasons change from spring to summer (Goldman 1991, 1999; Perret and Aujard 2001). Therefore, we aimed to determine whether the reproductive cycles of both female and male northern fur seals would be altered under experimentally induced conditions of extended photoperiods. Seasonal changes in gonadal steroid hormone levels and body weight were used as indicators of the reproductive cycle indices, because changes in these factors in relation to the reproductive cycle have previously been described (Kiyota et al. 1999; Kohyama et al. 1999). The effects of photoperiod on seasonal changes in reproductive cycles