The paper describes a comprehensive assessment of lung growth and biochemical maturation in the fetal and early postnatal rat. Fetal lung grew faster than whole body between gestational day 16.5 and term. Cell number increased quasi-exponentially, except for a slowing of cell growth between days 19.5 and 20.5 of gestation. By contrast, growth was limited during the two first postnatal days. Percentage of dry tissue and protein concentration increased in parallel during the whole period. Increases in whole lung tissue phosphatidylcholine were greatest between 20.5 and 21.5 days of gestation, whereas in isolated surfactant fraction, changes were most marked between 19.5 and 20.5 days. The most striking changes were a 19-fold increase in phosphatidylcholine and a 12.5-fold increase in the total phospholipid concentrations in the surfactant fraction between gestational day 19.5 and 1 day postpartum. Similar changes occurred in disaturated phosphatidylcholine (DSPC) concentration. During the same period, phospholipids of the residual (nonsurfactant) fraction increased only about 2-fold. These data indicate that analysis of an isolated surfactant fraction is advantageous in providing a very sensitive index of augmented phospholipid production during the process of fetal lung maturation. Evidence of biochemical maturation was detected earlier in females than in males, as indicated by a significantly larger surfactant fraction DSPC concentration in 19.5-day-old females; however, this difference was modest in degree and very transient, since it was no longer demonstrable in later stages.