Most previous studies of the relationship between surfactant and growth have correlated surfactant pool size with lung weight or body weight. We asked how the quantities of the surfactant components surfactant protein A (SP-A) and saturated phosphatidylcholine (Sat PC) recovered by alveolar wash changed with age in relationship to morphologic measurements of alveolar surface area in rats. We also calculated SP-A and Sat PC in total lung per surface area. In groups of three to 14 rats studied on d 1, 7, 14, 21, 50, and 100 of age, the ratio of alveolar surface area to body weight was highest at d 14 and lowest at d 100. Alveolar and total lung SP-A and Sat PC were higher relative to body weight and surface area on d 1 than at all other ages. Alveolar and total lung SP-A peaked relative to surface area at d 21 and subsequently decreased significantly at d 100. The alveolar Sat PC to surface area ratio also was highest at d 21 and decreased significantly at d 50 and 100. The SP-A to Sat PC ratio in alveolar washes was not constant across this age range, being highest at d 21 and 50. These measurements demonstrate age-dependent changes in alveolar and total lung SP-A and Sat PC in the rat that continue beyond the newborn period.