Early postnatal nutrition has been associated with the long-term effects on glucose homeostasis in adulthood. To elucidate the molecular mechanisms by which undernutrition during early life leads to changes in insulin sensitivity, we investigated the insulin signaling in skeletal muscle of rats during development. Offspring of dams fed with either protein-free or normal diets during the first 10 days of lactation were studied from lactation period until adulthood. Early maternal undernutrition impaired secretion of insulin but maintained normal blood glucose levels until adulthood. Insulin receptor (IR) activation after insulin stimulation was decreased during the period of protein restriction. In addition, glucose uptake, insulin receptor substrate 1 (IRS-1) phosphorylation and glucose transporter 4 (GLUT-4) translocation in muscle were reduced in response to insulin during suckling. In contrast, non- or insulin-stimulated glucose uptake and GLUT-4 translocation were found significantly increased in muscle of adult offspring. Finally, basal association of phosphatidylinositol 3-kinase (PI3-kinase) with IRS-1 was increased and was highly stimulated by insulin in muscle from adult rats. Our findings suggest that early postnatal undernutrition increases insulin sensitivity in adulthood, which appears to be directly related to changes in critical steps required for glucose metabolism.