Background: It has been hypothesized that increased fat oxidation reduces glucose utilization in skeletal muscle, and is responsible for the insulin resistance associated with obesity or high-fat feeding. In contrast, there have been reports that fat oxidation capacity was decreased in skeletal muscles from insulin resistant subjects. This study was undertaken to examine whether insulin resistance in high- fat fed rats is associated with increased lipolysis in skeletal muscle and adipose tissue. Methods: Two groups of Sprague-Dawley rats were fed either high-fat or low-fat diets for 4 weeks. Lipolysis in skeletal muscle and adipose tissue was determined by measurement of interstitial glycerol concentrations by a microdialysis method in basal and hyperinsulinemic-euglycemic clamp conditions. Results: In basal state, plasma glycerol levels and interstitial glycerol concentrations of skeletal muscle, and adipose tissue were lower in high-fat fed than in low-fat fed rats. The degree of suppression of glycerol release by the hyperinsulinemia was smaller in the high-fat diet than in the low-fat diet group. However, plasma and interstitial glycerol concentrations during the hyperinsulinemic euglycemic clamps were also lower in the high-fat diet group. Conclusion: Lipolysis was decreased in skeletal muscle and adipose tissue of insulin resistant, high-fat fed rats. These results support the idea that limited fat oxidation capacity resulting in lipid accumulation in tissues, rather than increased fat oxidation per se, is responsible for the insulin resistance associated with high-fat feeding(J Kor Diabetes Asso 24:641∼651, 2000).