Placing a translucent diffuser over the eye of a chick causes the eye to grow excessively, resulting in form-deprivation myopia. For chickens kept on a 12:12 h light/dark cycle, removing the diffuser for 3 h during the light period protects against the excessive growth, but if the bird is kept in the dark for this 3-h period, the protective effect is abolished. Injecting dopamine agonists into the eye during this 3-h dark period restores the protective effect, which can be blocked by dopamine antagonists injected just prior to diffuser removal in the light. These responses are mediated by D2 receptors, suggesting that the protective effect of normal vision against form-deprivation is mediated through the stimulation of dopamine release and activation of D2-dopamine receptors.