Purpose.The proliferation of many cell types are regulated by cytokines and neuropeptides by autocrine and paracrine mechanisms. Retinal pigment epithelial (RPE) cells are also regulated by cytokines. But RPE cells are very close to the neural retina which has some neuropeptides. The present study was to investigate the effects of neuropeptides on the growth of RPE cells.Methods, RPE cells were obtained from the eyes of 11 day old chick embryos and cultured in Dulbecco's modified Eagle's culture medium containing 10% fetal calf serum. The growth of RPE cells was evaluated by [3H]-thymidine uptake.Results.Substance P, p-endorphin and calcitonin gene-related peptide markedly stimulated the growth of RPE cells. The effects of methionine-enkephalin, somatostatin and vasoactive intestinal peptide were intermediate. The strongest effects of substance Rβ-endorphin and calcitonin gene-related peptide were observed at 106to 107M. The stimulation of RPE cells withβ-endorphin was inhibited by naloxone, suggesting that the stimulation withβ-endorphin is mediated by an opioid receptor.β-endorphin and substance P induced RPE cell growth stimulating activity. Leucine-enkephalin and neuropeptide Y did not affect the growth of RPE cells.Conclusions.These results suggest that neuropeptides play an important role in the regulation of RPE cell growth.