The motor zone of the rat brain cortex is subjected to pulsed infrared (0.89 ~t) laser radiation, which is found to stimulate DNA synthesis both in intact animals and after strenuous physical exercise (swimming). Preliminary laser irradiation exerts a stress-limiting effect on ceils of the brain cortex and thymus but does not prevent swimminginduced reduction of 3H-thymidine incorporation in nuclear DNA of muscles.