The early steps of insulin receptor (IR) signaling (tyrosine phosphorylation of IR beta-subunit, IRS-1 and Shc and PI 3'-kinase activity) have been characterized in two target tissues in the chicken: liver and muscle. The signaling cascade appeared to depend on nutritional status in the liver, but not in muscle (with a possible exception for a minor tyrosine phosphorylation of the 52 kDa Shc isoform). In this study, we compared the responses of the liver and muscle to exogenous insulin (10 or 1000 mU/kg) in chickens and rats. In the liver, IRS-1 and Shc proteins were present in smaller amounts and the regulatory subunit p85 of PI 3'-kinase was present in larger amounts in chickens than in rats. In the basal state (saline injection), the level of tyrosine phosphorylation of IR was lower, and that of Shc higher, in chickens than in rats. PI 3'-kinase activity in chickens was half that in rats. Insulin activated all components of the cascade in a dose-dependent manner in both species. A different pattern was observed in the muscle. In the basal state, the levels of tyrosine phosphorylation of IR and of PI 3'-kinase activity were much higher in chickens than in rats (by factors of 2 and 30, respectively). Insulin strongly activated all components of the cascade in rats (but with no significant increase in the phosphorylation of Shc). No activation was observed in chickens (with only a slight but significant increase in the tyrosine phosphorylation of Shc). The insulin cascade therefore appears to respond normally in chicken liver but to be refractory in chicken muscle. The large amount of p85 and high levels of PI 3'-kinase activity in muscle may contribute to this situation, making chicken muscle an interesting model of insulin resistance.