Four-week-old rats were divided into five groups: general controls, weight-matched controls (weight matched to hypobaric hypoxia), hypobaric hypoxia, normobaric hypoxia, and hypobaric normoxia. Lung growth impairment in weight-matched animals occurred by reduction in cell number and size. In both hypoxic groups, lung weight, RNA and protein were significantly higher on day 3, and DNA on day 5 and remained higher thereafter. Maximum 3H-TdR incorporation occurred on day 3 in both hypoxic groups. Hypoxia increased RNA/DNA ratio on day 1 and protein/DNA on day 3. Following 3 days of recovery, DNA synthesis and RNA/DNA ratio of hypoxic groups and controls were identical. DNA synthesis also doubled on day 5 in hypobaric normoxia compared to general controls. Hypoxia up regulates lung growth despite down regulation by undernutrition. Maximum lung growth stimulation occurs during early exposure by cellular hypertrophy followed by hyperplasia. Low pressure by itself also stimulates lung growth. Cellular activity returns immediately to normal levels after removal of hypoxic stimulus.