The reproductive and thyroid status of male Syrian hamsters maintained on long days (14 h light, 10 h darkness) were assessed after 10 weeks of daily injections of pharmacological doses of melatonin (25 micrograms s.c.) and/or N-methyl-DL-aspartic acid (NMDA, 0.025-6 mg i.p.), a compound with receptor sites in the central nervous system which are known to affect reproduction. Melatonin given during the late light phase decreased reproductive organ weights and levels of serum and pituitary prolactin and serum thyroxine (T4); these results are similar to published reports on the effects of chronic short photoperiod treatment of this species. Reproductive organ weights, T4 levels and values for prolactin did not differ significantly between groups receiving only melatonin and those receiving NMDA in addition to melatonin; likewise these variables did not differ significantly between groups receiving only either NMDA or saline. NMDA alone and in combination with melatonin increased serum tri-iodothyronine (T3). The brown adipose tissue enzyme T4 5'-deiodinase demonstrated an increased activity in the presence of NMDA, with the lowest dosage eliciting the most significant effect. Previous studies have demonstrated that NMDA reverses the reproductive effects of short photoperiod. The results of this study show that NMDA is incapable of preventing the inhibitory reproductive effects of exogenously administered melatonin. These observations are consistent with the proposal for a site of action for NMDA on neural regions more proximal than those altered by melatonin; alternatively, NMDA may interfere with neurotransmitter actions in the pathway controlling melatonin production.