A daily peroral administration to guinea pigs of solutions of nickel chloride over a period of from 5 to 7 days in doses of 0.5 microgram/kg/d increases the activity of cytochrome oxidase and succinate dehydrogenase in the subcortical part of the brain, liver and cortex of the kidneys and, in a dose of 20 microgram/kg in the muscles. The use of nickel in doses of 32 mg/kg and 1.28 g/kg results in inhibition of the activity of both enzymes in the cortex of the kidneys, and of cytochrome oxidase in the liver and spleen, as well as to an increase in the activity of cytochrome oxidase in the muscles. The effect of nickel on the activity of the enzymes in the different organs manifests itself in different ways. The maximum biotic effect is observed in the kidneys and muscles, and the maximum toxic effect in the kidneys, and in the spleen when large doses are used. The change in enzyme activity caused by nickel is better defined in succinate dehydrogenase than in cytochrome oxidase.