We have investigated the role of PI 3‐kinase and mTOR in the degradation of IRS‐1 induced by insulin. Inhibition of mTOR with rapamycin resulted in approximately 50% inhibition of the insulin‐induced degradation of IRS‐1. In contrast, inhibition of PI‐3 kinase, an upstream activator of mTOR, leads to a complete block of the insulin‐induced degradation. Inhibition of either PI‐3 kinase or mTOR prevented the mobility shift in IRS‐1 in response to insulin, a shift that is caused by Ser/Thr phosphorylation. These results indicate that insulin stimulates PI 3‐kinase‐mediated degradation of IRS‐1 via both mTOR‐dependent and ‐independent pathways. Platelet‐derived growth factor (PDGF) stimulation leads to a lower level of degradation, but significant phosphorylation of IRS‐1. Both the degradation and phosphorylation of IRS‐1 in response to PDGF are completely inhibited by rapamycin, suggesting that PDGF stimulates IRS‐1 degradation principally via the mTOR‐dependent pathway. Inhibition of the serine/threonine phosphatase PP2A with okadaic acid also induced the phosphorylation and degradation of IRS‐1. IRS‐1 phosphorylation and degradation in response to okadaic acid were not inhibited by rapamycin, suggesting that the action of mTOR in the degradation of IRS‐1 results from inhibition of PP2A. Consistent with this, treatment of cells with rapamycin stimulated PP2A activity. While the role of mTOR in the phosphorylation of IRS‐1 appears to proceed primarily through the regulation of PP2A, we also provide evidence that the regulation of p70S6 kinase phosphorylation requires the direct activity of mTOR. J. Cell. Biochem. 85: 304–314, 2002. © 2002 Wiley‐Liss, Inc.