SummaryLungs were obtained from rabbit fetuses (on each day from d 24 to d 30 of gestation), neonates and adults, and were fractionated for enzyme assays. The developmental profile of cytidyltransferase shows a decrease in specific activity from d 25 to d 29 (P<0.05) then a sharp rise from d 30 to adult values in d 0 neonates (P<0.05). Cholinephosphotransferase specific activity changes little from d 25 to birth, apart from a non-significant peak on d 29. There is a sharp rise from neonatal d 0 to adult values on d 1 (P<0.01). The specific activity of microsomal phospholipase A2declines from d 25 to reach adult values in the neonate (P=0.05). In contrast, the specific activity of lysosomal phospholipase A2rises from d 24-28 then falls in the neonate (P<0.05). Adult values are higher than those in the fetus and neonate. Three other lysosomal enzyme specific activities rise to d 28 then decline: phospholipase A1,β -gaIactosidase, and β - glucuronidase. The results demonstrate that the level of microsomal phospholipase A2does not control the extent of remodelling of phosphatidylcholine for surfactant production. Lysosomal phospholipase A2only increases in parallel with the other lysosomal enzymes, indicating an increase in the number of lysosomes in the lung.