Unoperated and partially hepatectomized rats were exposed to 100% O2 at 760, 258, and 191 mm Hg for various time periods up to 34 hr. No change in mitotic activity was noted in these animals when compared to appropriate controls. Similar groups of rats were exposed to a hypoxic condition (air, 380 mm Hg) for the same time periods as above. The partially hepatectomized group showed a delay in the initiation of mitosis, while the unoperated group showed a reduction in mitotic activity due to the hypoxic environment.