• Eight rhesus monkeys were photocoagulated with varying intensities of argon laser energy to determine the effects on the retinal nerve fiber layer. Photocoagulation of arterioles and venules always resulted in largely irreversible and permanent perivascular axonal destruction. Photocoagulation of areas free of major vessels showed variable responses to identical dosages, ranging from minimal destruction of the outer part of the retina to full-thickness destruction involving the nerve fiber layer. Variations in retinal thickness, vascularity, pigment epithelial pigmentation, and focus of the laser beam are responsible for differences in severity of each burn. Even when major vessels are avoided, it is impossible to predict accurately the degree of damage. However, our studies suggest that much of the initial damage is extracellular, sparing nerve fiber layer axons. Therefore, some acute damage might be reversible following resorption of the edema.