Studies of laboratory animal retinal exposure to green monochromatic laser radiation show an increasing stability of tissue with increasing pulse duration up to 10(-1) s. This mechanism of biological response of the eye to light damage is fundamental to living systems. It does not manifest itself in animals under deep anesthesia. A mathematical model has been developed which permits one to predict the dependence of threshold injury of visible monochromatic radiation. The model is based on the assumption that the entropy of the reaction of thermal tissue denaturation decreases with increasing pulse duration. This may be used to refine safety standards for personnel working with lasers and to optimize the parameters of lasers used in ophthalmic surgery.