Biliary excretion of cadmium was studied in rats after intravenous injection of different doses of cadmium chloride (0.1-2 mg Cd/kg). The rate of bile flow was not affected by cadmium injection and cadmium was excreted into bile during the first 2 hr after injection. The biliary excretion of cadmium increased with increasing dose of CdCl2. Cumulative biliary excretion of cadmium for 5 hr was 0.065% of the administered dose for groups injected with 0.1 mg Cd/kg as compared to 16.9% of the administered dose for 2 mg Cd/kg. During the 5 hr experimental period, most of the cadmium in liver cytosol was bound to high-molecular-weight proteins and less than 10% was bound to the metallothionein fraction. The biliary cadmium was recovered as a low-molecular-weight compound (less than 4,000) in experiments with various doses of cadmium and no cadmium was attached to high-molecular-weight proteins or metallothionein in the bile. The low-molecular-weight cadmium complex in bile was partially characterized as Cd-glutathione by thin-layer chromatography and amino acid analysis.