Insulin induces a wide variety of growth and metabolic responses in many cell types. These actions are initiated by insulin binding to its receptor and involve a series of alternative and complementary pathways created by the multiple substrates of the insulin receptor (insulin receptor substrates [IRSs]). We investigated IRS-1 and IRS-2 tyrosine phosphorylation; their association with phosphatidylinositol-3-OH kinase (PI3-K); and the phosphorylation of Akt, a serine-threonine kinase situated downstream of PI3-K, in liver and muscle of two animal models of insulin resistance: epinephrine- or dexamethasone-treated rats. We used in vivo insulin infusion followed by tissue extraction, immunoprecipitation, and immunoblotting. IRS-1 and IRS-2 protein expression did not change in liver and muscle of the epinephrine- treated rats, but in dexamethasone-treated rats IRS-1 presented an increase in liver and a decrease in muscle tissue. PI3-K and Akt protein expression did not change in liver or muscle of the two animal models of insulin resistance. There was a downregulation in insulin- induced IRS-1 and IRS-2 tyrosine phosphorylation and association with PI3-K in both models of insulin resistance. In parallel, insulin-induced Akt phosphorylation was reduced in both tissues of epinephrine-treated rats, and in liver but not in muscle of dexamethasonetreated rats. The reduction in insulin-induced Akt phosphorylation may help to explain the insulin resistance in liver and muscle of epinephrine-treated rats and in the liver of dexamethasone-treated rats.21115-22