This study defines the effects of fasting (prolongation of an overnight fast for a further four hours), feeding (the response to eating the three main ;solid' meals of the day), and cholecystokinin-induced gallbladder contraction (75-100 units of CCK given as a bolus intravenous injection) on serum individual bile acids in five to eight healthy control subjects. The serum conjugates of the two primary bile acids, cholic and chenodeoxycholic, were measured using sensitive specific radio-immunoassays. During fasting, there was no significant change in the levels of the serum individual bile acids (conjugates of cholate, 1.28 +/- 0.19; conjugates of chenodeoxycholate, 1.17 +/- 0.17 mumol/l). After breakfast, the serum conjugates of cholate and chenodeoxycholate increased significantly but thereafter the mean values remained high with less consistent responses to lunch and dinner, some subjects showing a peak and trough response to all three meals, while others showed a plateau response throughout the day. After breakfast, the serum chenodeoxycholate conjugates increased more rapidly (peak at 60 minutes when the concentration reached 2.07 +/- 0.30 mumol/l) and to a greater extent than the conjugates of cholate (peak at 90 minutes; 1.50 +/- 0.24 mumol/l). A similar pattern of results was seen after intravenous CCK, suggesting either preferential jejunal absorption of chenodeoxycholate conjugates and/or preferential hepatic clearance of cholate conjugates. These results provide essential background data for future studies of serum individual bile acids in intestinal and hepatic disease.