Newborn mammals in chronic hypoxia or hyperoxia experience, respectively, an increase or decrease in lung weight:body weight ratios, possibly because of the mechanical effect on the lung accompanying the ventilatory response. Because the avian lung does not expand or contract with the breathing cycle, we asked whether or not qualitatively similar changes could be observed in the lung of chick embryos incubated in hypoxic or hyperoxic conditions. Hypoxic embryos (10% O2, days 14-18) were smaller than controls incubated in normoxia, with higher hematocrit values and larger lung weight:body weight ratios (both wet and dry). Both the total pulmonary DNA (reflecting the cellular component) and the DNA concentration were decreased in hypoxia. Hyperoxic embryos (50% O2, days 7-18 or days 14-18) had lower hematocrit values and smaller dry lung weight:body weight ratios than controls, with similar DNA concentrations. In general, the differences from controls were more apparent in those embryos hyperoxic from day 14 to 18 of incubation than from day 7 to 18. We conclude that changes in lung weights qualitatively similar to those occurring in the chronically hypoxic or hyperoxic newborn mammal can also be observed in the hypoxic or hyperoxic chick embryo, suggesting that they are not necessarily caused by changes in mechanical stretch on the lung.