Purpose: Melatonin, secreted by the pineal gland at night, inhibits pubertal development in rats and possibly humans. We have recently found functional specific binding sites for 125I-labeled melatonin (125I- melatonin) in human benign prostate tissue, localized in the microsomal fraction of the glandular epithelium. The aim of the present study was to set up an animal (rodent) model for the growth inhibitory effects of melatonin on the prostate. Materials and Methods: Putative melatonin receptors in the rat ventral prostate were explored by means of autoradiography and receptor binding assays and the ability of melatonin to inhibit stimulated prostate growth was tested in vivo. Results: In vitro autoradiography and equilibrium binding experiments demonstrated specific binding sites for 125I-labeled melatonin (125I-melatonin) associated with the microsomal fraction of the rat ventral prostate cells (apparent dissociation constant 0.9 nM). 125I- melatonin binding was inhibited by 2-iodomelatonin> 6-hydroxy- melatonin>melatonin=N-(2,4 dinitrophenyl)-5-methoxytryptamine whereas similar concentrations of serotonin, 5-methoxytryptamine, and tryptamine were less potent. The guanine nucleotide analogs, guanosine 5'-0-[3-thio-triphosphate] and guanosine 5'-0-[2-thio-diphosphate], inhibited specific 125I-melatonin binding whereas 5'-guanylyl imido-diphosphate was less potent. Daily injections of testosterone to castrated rats induced regrowth of the prostate and increased the weight of the seminal vesicles. Administration of melatonin to the rats through drinking water prevented the testosterone-mediated regrowth of the prostate but had no effect on the seminal vesicles' weight. Conclusions: The results demonstrate putative melatonin receptors in the rat prostate and suggest a direct suppression by melatonin of testosterone- dependent prostate growth.