The relationship between insulin binding and its biological effects was studied in soleus muscle of 3- to 15-wk-old genetically obese (ob/ob) mice. At 3 and 4 wk of age, soleus muscle from lean and obese mice bound similar amounts of insulin under equilibrium binding conditions. However, by 6 wk of age, insulin binding and total receptor concentration (Ro) were significantly decreased in soleus muscle from obese compared to lean mice. In addition lean and obese mice demonstrated an age-dependent decrease in insulin binding, Ro, and receptor affinity. At 4 wk of age, insulin-stimulated 2-deoxyglucose transport and glucose utilization were significantly lower in soleus muscle from obese mice and preceded alterations in insulin binding. The postmembrane decrease in insulin sensitivity was dissimilar for various pathways of glucose metabolism. Glucose conversion of glycogen, but not the glycolytic rate or glucose oxidation, remained sensitive to insulin stimulation. These data indicate that glucose transport and utilization rather than insulin binding may play the primary role in the development of insulin resistance of muscle in the obese diabetic syndrome.