The effect of chronic hypoxic exposure on lung development has been assessed in growing guinea pigs (Cavia porcellus). Weanling males of initial W = 229 g were acclimated to a PO2 of 80 Torr for 2-14 weeks before sacrifice (range of W = 244-965 g). Growth was the same in hypoxic animals as in controls maintained at a PO2 of 133 Torr (range of W in controls = 89-1274 g). Lungs were fixed by tracheal instillation of glutaraldehyde and examined morphometrically with the electron microscope. Within 3 weeks of exposure, lung volume (VL) and alveolar surface area (Sa) were significantly increased by 32% and 27% respectively in the hypoxia acclimated animals compared to controls of similar W. However, these differences were progressively reduced with increasing time of exposure, and mean values of VL and Sa were not different between groups when W greater than 900 g. Chronic hypoxia accelerated lung development towards normal adult dimensions to a degree remarkably similar to that reported in cold acclimated guinea pigs. These findings are compatable with the theory of adaptive lung growth mediated by increased pulmonary blood flow, and suggest anatomical limitations to such growth related to an animal's age.