Thiazolidinedione (TZD) insulin sensitizers are specific agonists of peroxisome proliferator activated receptor (PPAR)g. However, their mechanism of action and the in vivo target tissue(s) that mediate insulin sensitization remain poorly defined. Although PPARg mes- senger RNA expression has been reported in skeletal muscle, the expression of PPARg within myocytes in intact muscle tissue has not been examined. An antipeptide PPARg antibody was generated; im- munohistochemistry was then used to demonstrate that PPARg is present within nuclei of myocytes (in both skeletal (white and red fibers) and cardiac tissue (rodent and human)). The effect of insulin sensitizer treatment on muscle insulin action was studied using ob/ob mice after 4 days dosing with a potent (6 nM PPARg Kd) TZD (10 mg/kgzday). 2-deoxyglucose (2-DOG) uptake was then assessed in freshly isolated soleus muscles from lean vs. ob/ob vs. TZD-treated ob/ob mice. In lean mouse muscles, 2-DOG uptake was stimulated by 82%, 95%, 165% (with 25, 100, 2000 mU/ml insulin); muscles from ob/ob were severely insulin resistant (,80% stimulation with 2000 mU/ml insulin). Muscles from TZD-treated ob/ob displayed a normal insulin response with 100 (71%) or 2000 (158%) mU/ml insulin. Ad- ditional studies were performed using ZDF rats treated with/without TZD for 7 days. In vivo 2-DOG glucose uptake into soleus, gastroc- nemius, and diaphragm muscles was measured during euglycemic- hyperinsulinemic clamp. Compared with lean rats, muscle 2-DOG uptake in ZDF was reduced by 52% (soleus) or 71% (diaphragm). Partial (40 - 60%) normalization of the reduced 2-DOG uptake was evident in TZD-treated ZDF rats. In contrast to the effect of in vivo treatment on muscle insulin action, preincubation of isolated soleus muscles from naive lean or ob/ob mice for 5 h with 100 nM TZD did not affect insulin-stimulated 2-DOG uptake. We conclude: 1) PPARg is expressed in myocytes within skeletal and cardiac muscle. 2) In vivo activation of PPARg by treatment of insulin-resistant mice/rats with a potent TZD corrects impaired muscle insulin action. 3) The lack of a direct effect on muscle after 5 h in vitro TZD incubation suggests that changes in insulin action may require a longer duration of PPARg activation or that improved muscle insulin sensitivity may result from an indirect in vivo effect of PPARg activation (e.g. changes in systemic lipid metabolism). (Endocrinology 139: 5034 -5041, 1998)