Evaluation of the effect of laser radiation on cat brains shows evidence of superficial destruction as well as deep penetration with the Q-switched pulse, while in the normal mode, irradiation up to 10 J of pulse energy produced no destructive changes. The indication, after 3 days, suggests a specificity of laser-produced lesion with a mechanism which would spare the deep white matter and produce periventricular lesion. The evidence of marked astroglial response, even after irradiation with normal mode, indicates the possibility of severe tissue alterations after a long survival period. The differential vulnerability of the cortex and the white matter also is illustrated in laser beam irradiation, since, even in the most severely damaged brains irradiated with Q-switched, 1-J. energy pulse where laceration of marginal cortex was achieved, the deep cortical areas show little or no alteration, while evidence of degeneration reappeared in the subcortical white matter. The techniques for enzyme histochemistry demonstrate cellular alterations in areas where other histological methods detect no change.