In rat adipocytes, insulin dose-response curves were determined for the following effects in the same cells under the same conditions: glucose uptake, binding to insulin receptors (IR), IR autophosphorylation in vivo and in vitro, IR tyrosine kinase activity and insulin-stimulated phosphatidylinositol (PI) kinase. All the EC50 values were essentially the same (mean +/- S.E.M. was 7 +/- 1 nM), except for glucose uptake, which was 170 pM. Using an improved method, we were able to measure PI kinase activity at picomolar concentrations of insulin (> 30 pM) corresponding to the EC50 for glucose uptake. These experiments showed that insulin-stimulated increase in glucose uptake was associated with an increase in antiphosphotyrosine antibody precipitable PI kinase activity, consistent with the view that IR tyrosine kinase activity may be involved in insulin-mediated signaling of glucose uptake. Small peptides (17-25 residues long) derived from major histocompatibility complex class I have previously been shown to inhibit IR internalization without any effect on the affinity of insulin to the receptor. It is now shown that the peptide-mediated inhibition of internalization, which doubles the number of insulin-occupied receptors at an insulin concentration of 70 pM, also results in a corresponding enhancement of PI kinase activity and glucose uptake. Thus, the receptors arrested on the cell surface by the peptide are biologically active.