In this study the effects of bile acids and other organic anions on the secretion of very-low-density lipoproteins (VLDLs) were evaluated in rat hepatocytes in primary culture. Incubation of cells with portal blood concentrations (10–200 μM) of bile acids resulted in dose-dependent suppression of secretion of VLDL-associated [³H]triglyceride (TG) formed from [³H]glycerol, and also of TG mass. The degree of the inhibition was highly correlated with intracellular bile acid concentration. Prolonged incubation with 100 μM extracellular taurocholic acid (TC) decreased the secretion of [³H]TG and TG mass to 35% and 50% of the controls respectively. Cellular content of mass and of [³H]TG during prolonged incubation with TC were about 20% and 60% higher than the controls respectively. The inhibitory effect remained for at least 24 h in the presence of TC without altering VLDL-lipid and VLDL-apolipoprotein compositions or the size distribution of the particles. Secretion of apoB-100 and of apoB-48 was inhibited to a similar extent. Cells largely lost their capacity to accumulate bile acids intracellularly after 48 h in culture. In these cells TC was unable to exert its suppressive effects. Taurine and glycine conjugates of all common bile acids were capable of suppressing [³H]TG secretion. Trihydroxylated (cholic acid) and various dihydroxylated (deoxycholic, chenodeoxycholic and ursodeoxycholic acids) bile acids had similar capacities in this respect, suggesting that their common sterol-3α-OH structure is required for the suppressive effect. Neither non-bile acid organic anions, e.g. bilirubin ditaurate and dibromosulphthalein, nor dianionic bile acid metabolites, e.g. sulphated taurolithocholic acid and lithocholate-3-O-glucuronide, showed any effect on [³H]TG secretion. These results indicate that bile acids might play a physiological role in regulating VLDL production by the liver, especially in the postprandial state when their enterohepatic circulation is stimulated.