The biliary maximum secretory rate (SRmax.) of glycoursodeoxycholate in the bile fistula anaesthetized rat was about five times that of unconjugated ursodeoxycholate. Likewise, the biliary SRmax. of tauro-7-ketolithocholate was more than three times that of unconjugated 7-ketolithocholate. The SRmax. of 7-ketolithocholate infused with taurine (to avoid depletion of the taurine pool) was still significantly lower than that of exogenous tauro-7-ketolithocholate. These data support the hypothesis that, for bile acids that are poorly water soluble, the maximal biliary secretion rate is dependent on conjugating capacity, which is the limiting step in the overall transport from plasma into bile.