The hepatic extraction ratio (Eh) of five common bile acids was determined by comparison of the area under the plasma concentration time curve (AUC) after administration into the femoral and portal veins of rats. Taurocholic acid (TCA), glycocholic acid (GCA), cholic acid (CA), deoxycholic acid (DCA) and chenodeoxycholic acid (CDCA) were studied. Using equimolar doses of the bile acids (9.3mumoles), the effects of both conjugation and the number and position of hydroxyl groups on Eh were compared. TCA had the highest Eh (about 80 per cent) followed in order by GCA (65 per cent), CA (55 per cent) and CDCA (55 per cent, with DCA exhibiting the lowest Eh (40 per cent). This suggests that conjugation has a more important effect on hepatic extraction of bile acids than the number of hydroxyl groups.