Production of free radicals of superoxide anion in tissues by cadmium, activities of superoxide dismutase and catalase to protect tissue damages caused by the free radicals and ATPase that plays an important role in energy metabolism at cellular level were investigated. Experiments in viro were conducted with liver, kidney and testicle tissue homogenates of rats adding 0.05~0.50 mM cadmium chloride, and in vivo experiments administering single dose of 5mg of cadmium/kg of body weight in 0.1% cadmium chloride solution intraperitoneally 48 hours prior to evisceration. Production of superoxide radicals in liver and testicle increased with addition of cadmium in vitro, but not in kidney. In vivo experiments, however superoxide radicals slightly increased in liver and kidney but not in testicle. Superoxide dismutase (Cu, Zn-SOD and Mn-SOD), catalase and ATPase (total, (Mg++)- & (Na+)- (K+)-) activity decreased in the presence of cadmium in dose dependent manner. Reduction of these enzyme activities varied not only with dosage of cadmium but also with type of tissue and between in vitro and in vivo experiment.