The influence of muscular contraction on the oxidative enzymes and the diameters of muscle fibers was investigated. Soleus muscles of guinea pigs were denervated for four weeks. The denervated fibers showed a reduction in the intensity of staining for beta-hydroxybutyrate dehydrogenase, cytochrome oxidase, succinate dehydrogenase, and NADH-dependent tetrazolium reductase. Denervation also resulted in a decrease in fiber diameter. Denervated soleus muscles were electrically stimulated to contract over a four-week period at a frequency normally received by slow contracting muscles. Electrical stimulation caused the stain intensity of histochemical reactions for oxidative enzymes to appear to be normal or greater than normal in 90% of the denervated fibers. Stimulation also caused 69% of the denervated fibers to be of normal or greater than normal size. The results demonstrate that contraction of denervated muscle by electrical stimulation prevents the loss of oxidative enzymes and the atrophy associated with denervation.