Insulin and IGF-I induced a similar stimulation of glucose transport in isolated soleus muscle. These actions require phosphatidylinositol (PI) 3-kinase activation since the PI 3-kinase inhibitor, wortmannin, blocked the stimulation by both peptides. We compared IGF-I with insulin in the ability to activate PI 3-kinase in the isolated soleus muscle from lean and gold thioglucose-induced obese insulin-resistant mice. In muscles from lean mice, IGF-I and insulin were able to activate PI 3-kinase with a similar time course, the effects being maximal within 3–5 min of stimulation. However, the IGF-I concentrations required to obtain similar effects on PI 3-kinase were about 10 times higher than the corresponding insulin doses. To determine through which receptor IGF-I was activating PI 3-kinase, the ability of IGF-I to activate both its own receptor and insulin receptor was simultaneously measured. Whatever the dose used (100 or 500 nmol/L), IGF-I activated to a nearly similar extent both the tyrosine kinase activity of its own receptor and that of the insulin receptor, suggesting that IGF-I was not only activating its receptor but was also able to stimulate the insulin receptor kinase. In muscles of obese insulinresistant mice, although the defect of PI 3-kinase activation in response to IGF-I was relatively less pronounced (45%) than in response to insulin (70%) when compared with lean mice, PI 3-kinase stimulation was still markedly altered in response to IGF-I.