Protein phosphatase (PP) 1 and 2A activities were measured in soluble and particulate fractions of rat lung homogenates, obtained from 18- to 22-day gestation fetal rats, from neonates, and from adults. PP 1 activity in the particulate fraction increased 7-fold from 15 ± 2 to 102 ± 12 (mean ± SD) U/g lung between 18 and 22 days gestation, increased a further 1.5-fold to 152 ± 18 U/g in newborns, with no further increase in adults. PP 2A activity in the particulate fraction increased 3-fold from 10 ± 2 to 32 ± 5 U/g between 21 and 22 days gestation and did not change postnatally. PP 1 activity in the soluble fraction increased 7.8-fold from 4 ± 2 to 31 ± 4 U/g lung between 18 and 22 days gestation, increased a further 1.8-fold in newborns to 54 ± 8 U/g lung and increased 1.5-fold to 81 ± 6 U/g lung in adults. PP 2A activity in the soluble fraction increased 2.2-fold from 12 ± 4 to 28 ± 8 U/g lung between 18 and 22 days gestation, increased a further 1.7-fold in newborns to 47 ± 9 U/g lung, and increased 1.6-fold to 75 ± 10 U/g lung in adults. PP 1 activity was 1.6-to 3-fold higher in the particulate than the soluble fraction. PP 2A activity was 2-fold higher in the soluble than the particulate fraction after 21 days gestation, in neonates and adults. We conclude that protein phosphatase activity is regulated developmentally during the critical perinatal period of fetal lung ontogeny and suggest that these enzymes play an important role in regulating phosphorylation reactions in the developing lung.