Lids were fused in six neonatal and one adult macaque monkey (Macaca mulatta and Macaca arctoides) and were kept fused for 1 to 18.5 months. The juvenile macaques, but not the adult one, developed myopia due to excessive elongation of the eye. In all animals, the immunohistochemical reactivity of the retina for vasoactive intestinal polypeptide (VIP) was much higher in the closed than in the open eyes. The neuropeptide was localized to the perikaryon and dendrites of amacrine cells. No difference was observed in substance P immunoreactivity between open and closed eyes, suggesting that the observed effect is selective. The change in VIP immunoreactivity could be the result of an increase in peptide synthesis, a decrease in peptide release, or a combination of the two. These results indicate that VIP may play a part in the regulation of postnatal ocular growth.