Mesenchymal stem cells (MSCs) are multipotent stem cells capable of differentiating into adipocytes in the presence of a hormone cocktail. These cells thus provide a promising model for studying the early events of adipogenesis. Here, we examine the involvement of the PI3K/Akt and mTOR/p70S6K signaling pathways in human MSC adipogenesis. We found that the two pathways were strongly activated with a similar temporal profile under the adipogenesis-inducing hormone cocktail and this activation could be blocked by LY294002, a specific inhibitor of PI3K. Furthermore, rapamycin, a specific inhibitor of mTOR, blocked the activation of mTOR/p70S6K but not PI3K/Akt. Both LY294002 and rapamycin severely suppressed lipid accumulation, as well as the expression of adipogenic markers, including PPAR gamma 2 and C/EBP alpha, two master adipogenic transcription factors. Together, these data indicate that the mTOR/p70S6K pathway acts downstream of the PI3K/Akt pathway in mediating the adipogenic conversion of MSCs. In conclusion, our data suggest that the PI3K/Akt and mTOR/p70S6K signaling pathways are essential for adipogenesis of human MSCs.