Male rats (25-26 days of age) housed with 14 hours of light per day (lights on 0600--2000 hours) were either olfactory bulbectomized (rendering them anosmic), bulbectomized plus pinealectomized (Pinx), or left intact. On the day following the operations, intact, anosmic, and anosmic-Pinx animals began receiving single, daily afternoon (1700--1800 hours) subcutaneous injections of 50 microgram of melatonin (MEL) for six weeks, while an additional group of intact controls received injections of diluent. At the end of this period, body, anterior pituitary, testicular, and seminal vesicle weights were significantly reduced in intact-MEL-treated animals. Anosmic animals that had been treated with MEL experienced a further, highly significant, 65%, 90%, and 85% depression in testicular, seminal vesicle, and ventral prostate weights, respectively, as compared with intact control and MEL-treated rats. Additionally, both body and anterior pituitary weights were significantly decreased in MEL-treated, anosmic rats. Anosmic-Pinx rats treated with MEL had organ and body weights that were intermediate between those of intact-MEL and anosmic-MEL-treated animals. Pituitary and serum levels of prolactin (Prl) were significantly lower in anosmic-MEL-treated rats than in intact-MEL-treated groups. Similarly, Prl levels were depressed in the anosmic-Pinx rats treated with MEL; however, serum Prl was not statistically lower than in intact or intact-MEL-treated animals. These results indicate that anosmic male rats have an increased sensitivity to antigonadotrophic and Prl-inhibitory effects of MEL. Futhermore, the data suggest that the presence of the pineal gland in anosmic rats is important in permitting anosmia maximally to sensitize the neuroendocrine-reproductive axis to the antigonadotrophic effects of exogenously administered MEL.