The serum levels of conjugates of chenodeoxycholic acid (chenyl conjugates) and of cholic acid (cholyl conjugates) were determined by specific radioimmunoassays during a 24-hour period, which included three liquid meals and an overnight fast, in five healthy volunteers, five patients with previous cholecystectomy, five patients with documented bile acid malabsorption because of ileal resection, and four pregnant women. In healthy subjects, fasting-state levels of chenyl conjugates, when compared with those of cholyl conjugates, were higher; postprandially, levels of chenyl conjugates rose to a peak sooner (30 minutes vs 60 minutes) and to higher levels (5.2 +/- 1.3 muM vs 2.0 +/- 0.5 muM, M +/- SE). In cholecystectomised patients, the integrated areas under the curve for both bile acids were similar to those of the healthy controls, but postprandial peaks were less marked. In patients with bile acid malabsorption, postprandial rises of chenyl conjugates were lower but remained relatively constant throughout the day, whereas cholyl conjugate levels diminished progressively with each successive meal, consistant with depletion of the cholyl, but not the chenyl, pool. In three of four pregnant women, the postprandial rise of chenyl conjugates was disproportionately less compared with that of healthy controls. These results confirm the dynamic complexity of serum bile acid levels in man and indicate that the major circulating primary bile acids are chenyl conjugates. They support previous proposals that jejunal absorption of chenyl conjugates is important in the normal enterohepatic circulation of bile acids; and they suggest an abnormality in the enterohepatic circulation in pregnancy.