This study investigated the roles of the melatonin signal and the circadian system in the induction of photoperiodic responses in the male Syrian hamster. Pinealectomized animals received programmed s.c. infusions of saline or melatonin. Saline infusions for 10 h or melatonin for 4 h during the night had no effect on the reproductive axis whereas nightly 10-h infusions of melatonin induced gonadal atrophy. Animals that received 10-h infusions of melatonin arranged such that consecutive daily signals were delivered alternately during the day and night also exhibited gonadal atrophy, whereas melatonin signals delivered every 48 h, exclusively during either the day or night, were without effect. These results demonstrate that the brain is able to read melatonin signals delivered at different phases of the circadian cycle and to use them in combination to generate an appropriate photoperiodic response. Melatonin signals lasting 10 h delivered to pinealectomized (PX) animals every 24 h induced gonadal regression. Melatonin delivered at periodicities of 20 h, 23 h, and 25 h also caused gonadal regression whereas infusions every 28 h were without effect, demonstrating that the systems responsive to melatonin are sensitive to signal frequency but do not need to receive the signal on a strictly circadian basis. These results are discussed in the context of the significance of the melatonin-free interval. PX animals that received sham or bilateral lesions of the suprachiasmatic nuclei (SCN) were infused nightly for 10 h with saline or melatonin. Melatonin infusions were equally effective at inducing gonadal atrophy and lowering serum testosterone levels in both sham- and SCN-lesioned animals.(ABSTRACT TRUNCATED AT 250 WORDS)