The interrelationships between biliary bile acid, lecithin and cholesterol secretion rates were studied druing depletion of the bile acid pool and during duodenal administration of cholic or chenodeoxycholic acid in thirteen patients 7-12 days after operation for uncomplicated gallstone disease. The mean lecithin secretion rate was signigicantly higher during cholic acid than during chenodeoxycholic acid infusion. The relationship between bile acid and cholesterol secretion rates was curvilinear, y = x/(a + bx) during bile acid pool depletion and during duodenal cholic acid infusion. At low bile acid secretion rates, during bile acid pool depletion and during cholic acid infusion, the lecithin secretion rate was significantly correlated to the cholesterol secreation rate. The bile acid and cholesterol secretion rates were not significantly correlated during chenodeoxycholic acid infusion. However, under this experimental condition a significant curvilinear relationship between lecithin and cholesterol secretion rates was found. The hepatic bile became unsaturated in cholesterol at significantly lower bile acid secretion rate during chenodeoxycholic acid infusion (10.7 +/- 0.3 mumol min-1) than during cholic acid infusion (15.6 +/- 0.5 mumol min-1).