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.(ABSTRACT TRUNCATED AT 250 WORDS)