Rats were exposed to crhonic hypobaric hypoxia at a simulated altitude of 4250 m for 3, 6, 9, 12, 20, and 35 days. The in-vitro incorporation of 3H-thymidine into the DNA of lung tissue was measured and compared with that of normoxic controls: the obtained time course study showed a maximum increase of 345% on the ninth day of hypoxia, indicating a marked stimulation of cellular proliferation. Between the 12th and 20th day of hypoxia, the lung DNA-synthesis reached control values. No significant change in the DNA-concentration of the lungs was registered. The response to hypoxia was less impressive in rat livers used as controls.