The mammalian target of rapamycin (mTOR) is a central regulator of cell growth. mTOR exists in two functional complexes, mTORC1 and mTORC2. mTORC1 is rapamycin-sensitive, and results in phosphorylation of 4E-BP1 and S6K1. mTORC2 is proposed to regulate Akt Ser473 phosphorylation and be rapamycin-insensitive. mTORC2 consists of mTOR, mLST8, sin 1, Protor/PRR5, and the rapamycin insensitive companion of mTOR (rictor). Here, we show that rapamycin regulates the phosphorylation of rictor. Rapamycin-mediated rictor dephosphorylation is time and concentration dependent, and occurs at physiologically relevant rapamycin concentrations. siRNA knockdown of mTOR also leads to rictor dephosphorylation, suggesting that rictor phosphorylation is mediated by mTOR or one of its downstream targets. Rictor phosphorylation induced by serum, insulin and insulin-like growth factor is blocked by rapamycin. Rictor dephosphorylation is not associated with dephosphorylation of Akt Ser473. Further work is needed to better characterize the mechanism of rictor regulation and its role in rapamycin-mediated growth inhibition. (C) 2007 Elsevier Inc. All rights reserved.