Abstract: Circannual variation in the human serum levels of prostate-specific antigen, a growth marker of the prostate gland, has been reported recently. The present study was conducted to investigate the role of the photoperiodic hormone melatonin (MLT) and its membrane receptors in the modulation of human prostate growth. Expression of MT1 and MT2 receptors was detected in benign human prostatic epithelial tissues and RWPE-1 cells. MLT and 2-iodomelatonin inhibited RWPE-1 cell proliferation and up-regulated p27Kip1 gene and protein expression in the cells. The effects of MLT were blocked by the nonselective MT1/MT2 receptor antagonist luzindole, but were not affected by the selective MT2 receptor antagonist 4-phenyl-2-propionamidotetraline. Of note, the antiproliferative action of MLT on benign prostate epithelial RWPE-1 cells was effected via increased p27Kip1 gene transcription through MT1 receptor-mediated activation of protein kinase A (PKA) and protein kinase C (PKC) in parallel, a signaling process which has previously been demonstrated in 22Rv1 prostate cancer cells. Taken together, the demonstration of the MT1/PKA+PKC/p27Kip1 antiproliferative pathway in benign and malignant prostate epithelial cell lines indicated the potential importance of this MLT receptor-mediated signaling mechanism in growth regulation of the human prostate gland in health and disease. Collectively, our data support the hypothesis that MLT may function as a negative mitogenic hormonal regulator of human prostate epithelial cell growth.