To investigate the difference of insulin action between skeletal muscle and adipose tissue in response to dietary manipulation, we studied the effect of high-sucrose (HS) and high-fat (HF) diet on insulin action by measuring insulin binding and insulin action both in soleus muscles and adipocytes. HS feeding led to a 14 and 28% decrease, and HF feeding led to a 25 and 36% decrease in insulin binding both to soleus muscles and adipocytes, respectively (P less than 0.01). In HF-fed rats, both rates of glucose uptake and intracellular glucose metabolism were impaired by 30-40% in soleus muscles (P less than 0.01), and adipocyte glucose uptake was also decreased by 30% in the submaximally insulin-stimulated state (P less than 0.05). On the other hand, in HS-fed rats with prominent hyperinsulinemia, glucose uptake was 2.3- to 2.7-fold increased in adipocytes (P less than 0.01). However, both rates of glucose uptake and intracellular glucose metabolism were not increased in soleus muscles from HS-fed rats. Steady-state plasma glucose (SSPG) level was unchanged in HS-fed rats during somatostatin, glucose, and insulin infusion, whereas the SSPG level of HF-fed rats was twice as much as that in controls (P less than 0.01). These results indicate that long-term regulation of glucose metabolism by ambient insulin in skeletal muscle may be different from that in adipocytes and that insulin action in muscle, rather than in adipocyte, may reflect insulin action of whole body.