After insulin binding, insulin receptors (IR) phosphorylate the insulin receptor substrate 1 (IRS-1) on specific motifs and thereby initiate insulin action. The interaction between IR and IRS-1 and their expression were studied in vivo in two target tissues (muscle and liver) in chickens, a species that is insulin resistant. To induce extreme changes in plasma insulin levels, chickens were subjected to three different nutritional states (ad libitum fed, fasted for 48 h, and refed for 30 min after 48-h fast). Liver membrane IR number was significantly increased in fasted compared with fed chickens. This upregulation of IR number was concomitant with the an enhanced expression of IR mRNA as determined by reverse transcription-polymerase chain reaction. In leg muscle, IR mRNA was not altered by the nutritional state. Using specific antibodies directed toward human IR, anti-phosphotyrosines, or mouse IRS-1, we demonstrated that IR and IRS-1 are associated in vivo in liver and muscles. Tyrosine phosphorylation of liver IR and IRS-1 were significantly decreased by prolonged fasting and restored by 30-min refeeding. These alterations were not observed in muscle. Fasting increased IRS-1 mRNA expression in liver but not in muscle. These results are the first evidence showing that chicken liver and muscle express IRS-1. Therefore, the chicken insulin resistance is not accounted for by the lack of IRS-1. The differences observed for the regulation of IR and IRS-1 messengers and phosphorylation between liver and muscle in response to alterations of the nutritional state remain to be explained.