During the last stages of intrauterine growth, remarkable changes occur in the alveolar epithelium that include cellular differentiation and increased production of surfactant lipid and apoprotein. We made morphometric measurements of type II cell characteristics from rats aged gestational day 20 to 14 days postnatal. We also measured the amounts of disaturated phosphatidylcholine (DSPC) and surfactant apoprotein (SP-A) in lung tissue, bronchoalveolar lavage, and a lamellar body-rich fraction, and we estimated the lung content of mRNAs for SP-A, SP-B, and SP-C. Lavage and lamellar body surfactant lipid and apoprotein content per lung showed a pattern of a sharp rise in the early postnatal period, then a substantial decline, and a second increase by day 14. When normalized for dry lung weight, the highest DSPC values were found on postnatal day 1 in all compartments. The fraction of whole lung DSPC found in lamellar body or lavage was greatest in the 48-h period surrounding birth. Lamellar body SP-A was greater than lavage SP-A on gestational day 22, but a day later the lavage SP-A was 16 times greater than the lamellar body SP-A. The lung tissue content of all three apoprotein mRNAs increased sharply before birth, fell during the 1st postnatal wk, and then rose again to adult levels. Type II cell number and lamellar body number per milligram of dry lung tissue was highest on post-natal day 1 and fell by one-half during the 1st postnatal wk. The amount of DSPC per unit of lamellar body volume rose to its greatest value on postnatal day 1 and then decreased more than threefold. These findings indicate a pattern of expansion of surfactant cellular and biochemical pools at the time of birth in the rat.