Rapamycin is a potent antiproliferative agent that arrests cells in the G1 phase of the cell cycle through a variety of mechanisms involving the inhibition of the mTOR pathway. The majority of normal cells in culture are sensitive to the cytostatic effects of rapamycin while the growth of many malignant cells and tumors is rapamycin resistant. We had shown previously that hepatic DNA synthesis in the late gestation rat fetus is rapamycin resistant even though signaling through the mTOR/S6K pathway is attenuated. On the basis of this finding, we went on to characterize the response to rapamycin in a spectrum of tissues during late gestation and the early postnatal period in the rat. We found that rapamycin had no effect on DNA synthesis in major organs such as heart, lung, and kidney in the fetal and early postnatal rat despite a marked attenuation in the phosphorylation of ribosomal protein S6. In contrast, the proliferation of mature hepatocytes during liver regeneration was highly sensitive to rapamycin. These data indicate that basal cellular proliferation in a wide variety of tissues is rapamycin resistant and occurs independently of mTOR/S6K signaling. Furthermore, the well characterized effects of rapamycin in tissue culture systems are not recapitulated in the asynchronous cell proliferation that accompanies normal growth and tissue remodeling.