We and others recently generated mice with a targeted disruption of the insulin receptor substrate 1 (IRS-1) gene and demonstrated that they exhibited growth retardation and had resistance to the glucose-lowering effect of insulin. Insulin initiates its biological effects by activating at least two major signalling pathways, one involving phosphatidylinositol 3-kinase (PI3-kinase) and the other involving a ras/mitogen-activated protein kinase (MAP kinase) cascade. In this study, we investigated the roles of IRS-1 and IRS-2 in the biological action in the physiological target organs of insulin by comparing the effects of insulin in wild-type and IRS-1-deficient mice. In muscles from IRS-1-deficient mice, the responses to insulin-induced PI3-kinase activation, glucose transport, p70 S6 kinase and MAP kinase activation, mRNA translation, and protein synthesis were significantly impaired compared with those in wild-type mice. Insulin-induced protein synthesis was both wortmannin sensitive and insensitive in wild-type and IRS-1 deficient mice. However, in another target organ, the liver, the responses to insulin-induced PI3-kinase and MAP kinase activation were not significantly reduced. The amount of tyrosine-phosphorylated IRS-2 (in IRS-1-deficient mice) was roughly equal to that of IRS-1 (in wild-type mice) in the liver, whereas it only 20 to 30% of that of IRS-1 in the muscles. In conclusion, (i) IRS-1 plays central roles in two major biological actions of insulin in muscles, glucose transport and protein synthesis; (ii) the insulin resistance of IRS-1-deficient mice is mainly due to resistance in the muscles; and (iii) the degree of compensation for IRS-1 deficiency appears to be correlated with the amount of tyrosine-phosphorylated IRS-2 (in IRS-1-deficient mice) relative to that of IRS-1 (in wild-type mice).