Changes in the duration of the nocturnal peak of the circadian rhythm of melatonin secretion by the pineal gland mediate pohotoperiodic control of reproductive cycles in mammals. The present study examined whether the tau mutation of the circadian clock of the Syrian hamster altered the ability of the animals to exhibit photoperiodic, gonadal responses to timed infusions of exogenous melatonin. Pinealectomized tau mutant and wild-type hamsters received s.c. infusions of vehicle or melatonin solution (50 ng/h) for either 8 h or 6.67 h (i.e., 83% of 8 h, a reduction by the same proportion as circadian period is shortened by the mutation, 20 h vs. 24 h) once every 20 h for 6 wk. Testicular weight and serum levels of LH and prolactin were measured to assess the effects of the infusions on the reproductive axis. Both genotypes exhibited significant gonadal regression to the 8 h and 6.67 h melatonin signals, versus saline controls, although in taus the magnitude of the response to 6.67 h melatonin was significantly greater and more consistent than in wild-types. Compared to saline-infused hamsters, animals that received infusions of melatonin of either duration showed significantly reduced serum levels of LH and prolactin. There was no significant difference between the two genotypes in the observed endocrine responses to melatonin infusions. These preliminary data demonstrate that tau mutant hamsters can respond to programmed systemic infusions of melatonin in a manner comparable to that observed in wild-type hamsters, and therefore, that the tau mutation of the circadian clock has not impaired the ability of the hamster to measure photoperiodic time, as represented by melatonin.