Insulin has a paradoxical effect on a 41kDa Gi-like protein: Although insulin-treatment of rat adipocytes inhibited pertussis toxin-catalyzed ADP-ribosylation of a 41kDa G-protein in membranes in a dose-dependent manner, it simultaneously increased ADP-ribosylation of a 41kDa G-protein that co-immunoprecipitates with the insulin receptor (GIR41). The latter effect was insulin concentration- and time-dependent. The dose-dependent stimulatory effect of insulin on the autophosphorylation of the insulin receptor and on the ADP-ribosylation of the GIR41 in the insulin receptor immunoprecipitates closely paralleled each other. The time course of insulin-stimulated increase in the ADP-ribosylated GIR41, although rapid, was slower than the autophosphorylation of the receptor. The GIR41 is associated with and regulated by the insulin receptor further supporting an important role for this G-protein in modulating insulin action at the receptor level.