In rats exposed to either 300 or 500 pulses/min no significant light-microscopic or electron-microscopic changes were observed. The activities of alkaline phosphatase, 5'-nucleotidase, isocitrate dehydrogenase, a-glycerophosphate dehydrogenase, lactate dehydrogenase, NADH-diaphorase and monoamine oxidase did not change significantly in brain or liver of animals exposed to either 300 or 500 pulses/min. Rats irradiated for lOmin with 500 pulses/min had, however, decreased glucose 6-phosphate dehydrogenase and succinate dehydrogenase activities around the central veins of the liver. ATPase activity was lower in the proximity of the bile canaliculi. Acid phosphatase was more diffuse in the cytoplasm of the irradiated liver than in that of the control. In the brain, however, no enzyme changes were observed in rats irradiated with either 300 or 500 pulses/min.