1. In oxidative (soleus) and glycolytic (extensor digitorum longus) muscles of obese Zucker rats, a significant decrease in the percentage of relative area occupied by glycolytic fibers was observed. 2. The activity of citrate synthase and beta-hydroxy-acyl-CoA-dehydrogenase was significantly higher in muscles of obese than of lean Zucker rats. 3. In rats, 6 weeks after lesion of the ventromedial hypothalamus, no changes were observed. 4. This indicates that neither the proportion of oxidative fibers, nor the oxidative capacities are decreased in skeletal muscles of obese rats suggesting that insulin resistance cannot be ascribed to a higher glycolytic-oxidative fiber ratio.