In rabbits and rats the excretion via the bile of the highly effective and selective synthetic thrombin inhibitor N alpha-(2-naphthylsulfonylglycyl)-4-amidinophenylalanine piperidide was studied after various routes of administration and varying doses. The concentration in the bile was determined using a biological method, the reliability of which was checked by HPLC-determinations. The compound was excreted in the bile in biologically active unchanged form. Cumulative biliary excretion of the synthetic thrombin inhibitor in rabbits amounted to a high percentage of the administered dose after low doses given either systemically or intraportally. In rats the excretion rates during and after intravenous infusion reached a maximum indicating saturation kinetics. Hepatic uptake and biliary excretion of the highly basic benzamidine derivative essentially contribute to its short plasma half-life.