Aldosterone metabolism has been shown to be altered in pregnancy. The increased conversion of intravenously administered aldosterone-(3)H to the acid-labile conjugate in the urine (aldosterone 18-glucuronide) has again been observed. The urinary yield of intravenously administered aldosterone-(3)H as aldosterone 18-glucuronide in 16 pregnant subjects of 13.4+/-0.9 (SE)% was significantly higher (P 0.001) than in 11 nonpregnant subjects (seven males and four females) of 7.3+/-0.5 (SE)%. After combining oral ((14)C) and intravenous ((3)H) administration of aldosterone, the (14)C/(3)H ratios of urinary metabolites (free aldosterone, tetrahydroaldosterone glucuronide, and aldosterone 18-glucuronide) were measured and were expressed as a per cent of administered dose. From these data the splanchnic extraction of aldosterone was calculated. The splanchnic extraction was significantly lower in pregnant as compared to nonpregnant subjects, although previous work indicated no change in protein binding of aldosterone in pregnancy.However, the data on the (14)C/(3)H ratio of other metabolites suggested that a large part of the increased aldosterone 18-glucuronide metabolite in pregnancy is formed in the splanchnic circulation; also, there appeared to be increased tetrahydroaldosterone glucuronide formation extrasplanchnically.