Melatonion concentrations were determined in plasma pools obtained from adult male Pteropus poliocephalus (autumn mating season) at four times of the year. Melatonin levels increased within 3 h of sunset and remained elevated for the duration of the scotophase at all times of the year. Two photoperiod manipulation experiments were performed to examine the role of daylength in the regulation of the timing of the breeding season of this species. In Experiment 1, three adult males were transferred from natural short days to 16L:8D for 137 days and then photoperiod was progressively decreased over 120 days to 9L:15D; this photoperiod was then maintained for 350 days. Testicular volume (TV) peaked during decreasing photoperiod well before the time of maximal size in natural conditions. During the period in extended short photoperiod these bats showed several cycles of TV change with a progressively decreasing interval between cycles. In Experiment 2, nine adult males were exposed to the same shift to 16L:8D as in Experiment 1, but were subsequently split into three groups: 8L:16D, progressive decrease to 8L:16D and maintenance in 16L:8D. The 8L:16D and decreasing photoperiod groups showed coincident premature increases in TV, as in Experiment 1, whereas in the 16L:8D group TV increased at about the same time as in animals in natural photoperiod. Melatonin determination in the different experimental groups showed that duration of secretion was related to the length of the scotophase in all cases. These experiments demonstrate that regulation of the timing of reproduction in males of this species is influenced by changes in daylength.