Metabolic activities of acoustical and somatosensory centers of the rat brain were investigated by revealing a mitochondrial enzyme, cytochrome oxidase (CO), with a histochemical method in control animals and those exposed to electromagnetic fields of a superhigh frequency (EMFSF). In treated animals an increase in the enzyme activity was found in the structures examined. The highest CO reactivity was observed in the brain areas involved in the processing of ecologically important sensory signals.