The effect of cholestyramine (12 gm/day divided into 3 doses) on the pharmacokinetics and pharmacodynamics of a single intravenouse dose (30 mg) of phenprocoumon was studied in 6 normal subjects. Cholestyramine treatment led to an increase in the rate of elimination of phenprocoumon in all. Total clearance increased 1.5- to 2-fold. The total anticoagulant effect per dose was considerably reduced during treatment with cholestyramine. Binding studies in vitro showed that phenprocoumon is strongly bound to cholestyramine and that at a given cholestyramine concentration the percentage of phenprocoumon bound remained constant over a large concentration range of phenprocoumon. The results suggest that phenprocoumon undergoes extensive enterohepatic recycling in man which can be effectively interrupted by cholestyramine.