Ames dwarf mice are a model of retarded aging and extended longevity and display enhanced insulin sensitivity. Caloric restriction (CR) and the dwarf mutation have additive effects on lifespan. To begin to understand the mechanisms behind this effect, an analysis of the in vivo status of the insulin signaling system was performed in skeletal muscle from Ames dwarf (df/df) and normal mice fed ad libitum or subjected to long-term (over 1 year) CR. The response to CR was different in both groups of animals. In normal animals, CR induced a significant reduction in both circulating insulin and glucose levels, together with an increase in the in vivo insulin-stimulated phosphorylation of the IR, a trend towards an increase in the in vivo insulin-stimulated phosphorylation levels of IR substrate-1, and an increase in the abundance of GLUT4 in muscle. In contrast, CR did not modify none of these parameters in df/df mice. Interestingly, CR induced a reduction in the p85 subunit of phosphatidylinositol 3-kinase abundance in skeletal muscle in both groups of animals. These results suggest that in skeletal muscle, long-term CR induces different effects on the first steps of the insulin signaling system in normal mice than in df/df mice.