Sex-related differences in lung maturity have been associated with lower surfactant production by the male fetus in late gestation, but the possibility that differences in lung growth occur between male and female lungs has not been examined in detail. Using rat littermates grouped according to sex from day 17 to 22 of gestation, we have now correlated proliferation of the various lung cell types with biochemical indices of cell function. Although the overall 3H-thymidine-labeling index in the lung decreases sharply in all animals approaching full term, the proportion of epithelial cell labeling increases from 30% at day 17 to 47% at day 20. During this period there are significantly more labeled epithelial cells in females than in male littermates. At the conclusion of this phase of epithelial proliferation, day 20 of gestation, the level of disaturated phosphatidylcholine increases sharply and is significantly greater in female lungs. Subsequently, the male fetus appears to "catch up," and sex-related differences in cell proliferation and surfactant levels are eliminated. The results suggest that differences in lung structure between males and females predate surfactant differences and are first observed during a period of epithelial proliferation. Completion of this phase results in earlier differentiation and surfactant synthesis in the female fetus.