The response of plasma renin activity (PRA) and plasma aldosterone (PA) to change in sodium intake was evaluated in pregnant women during the third trimester. After 7 days on a 10 mEq sodium diet, PRA rose from 20.6 +/- 6.2 to 59.6 +/- 11.6 ng/ml/hr and PA from 47 +/- 11 to 127 +/- 27 ng% in pregnant women compared to PRA from 5 +/- 1.2 to 18.9 +/- 5.2 ng/ml/hr and PA from 7.7 +/- 1 to 42 +/- 3 ng% in nonpregnant controls. Pregnant women conserved sodium normally with urinary sodium excretion and weight loss similar to nonpregnant women. After 6 days on a 300 mEq sodium intake, PRA and PA in pregnant women were significantly higher, 10.2 +/- 1.4 ng/ml/hr and 22 +/- 3 ng%, respectively, compared to 1.5 +/- 0.3 ng/ml/hr and 7.3 +/- 1 ng% in controls. On both low- and high sodium intake there was a positive correlation between PRA and PA in pregnant women. Plasma prostaglandin E (PGE) was 0.45 +/- 0.06 ng/ml in pregnant women compared to 0.1 +/- 0.01 ng/ml in control women (p less than 0.01) and urinary PGE excretion was 2780 +/- 357 ng/24 hr in 28 pregnant women compared to 1191 +/- 142 ng/24 hrs (p less than 0.01) in 14 nonpregnant controls. These findings indicate that although renin and aldosterone secretion respond to change in sodium intake in pregnancy, the cause of the increased renin secretion of pregnancy may be secondary to the increase that occurs in prostaglandin synthesis.