The response of total nonsulfated serum bile acids, cholylglycine, and chenodeoxycholyl species was examined every 20 min for 3 h in 6 subjects. Noncaloric feeding led to a progressive decline or no change in bile acids, while there was a progressive rise in response to a standard liquid meal. After reaching a peak at 60 min, total bile acids declined progressively but cholylglycine and chenodeoxycholyl species remained elevated. Continuous infusion of cholecystokinin led to significantly greater levels most probably due to more rapid enterohepatic recirculation. Oral administration of 250 mg of chenodeoxycholic acid with water resulted in a rise in total bile acids and chenodeoxycholyl species, but not cholylglycine, indicating the rise was due to the administered bile acid and not gallbladder contraction. Administration of a meal and chenodeoxycholic acid simultaneously caused no greater rise of total serum bile acids or cholylglycine than either stimulus alone. Peak response and area under the curve were compared for each patient. The increase for chenodeoxycholyl species was additive for the two stimuli, suggesting that free chenodeoxycholic acid, when administered with a meal, decreased the absorption of endogenous conjugated bile acids. This study is compatible with the thesis that serum bile acids accurately reflect enterohepatic cycling and that administration of chenodeoxycholic acid with a meal may decrease its efficacy because exogenous chenodeoxycholic acid may compete with endogenous bile acids for absorption.