Effects of focused ruby laser beams on the cerebral cortex of dogs were studied. Energy levels were tested from 5- to 40-joule exit energy with convergent, surface-focused and divergent beams from the liquid nitrogen-cooled ruby laser. Discrete and reproducible lesions could be induced. Possible mechanisms of laser effect in tissues include thermal injury, as determined by histopathologic criteria, and local anoxia caused by vascular injury. Laser effects in biologic systems are probably determined to a significant extent by the local vascularity and, of course, by pigmentation of the tissues. Laser effects on the cellular elements of the cerebral cortex were not uniform, the neurons being very susceptible to laser energy whereas astrocytes proved more resistant. However, the local vascular bed was severely damaged by laser, and it is possible that the apparent difference in cell susceptibility to laser energy may be secondary to the local anoxia.