The absorption and metabolism of the disulfide dimer conjugate of captopril has been studied in the rat following both oral and intravenous dosing and compared with that of the active monomer, captopril. Metabolism of the dimer to captopril has been shown after both oral and intravenous administration of the dimer (10 mg/kg) with peak plasma levels of captopril (154 ng/ml) occurring at 1 hr post dose. By contrast the peak plasma level of captopril after oral administration of captopril (10 mg/kg) at the same dose was much higher at 678 ng/ml and also occurred at 1 hr post dose. Plasma captopril disulfide species were much higher than the plasma levels of captopril after the administration of either dimer or captopril and tended to persist for much longer than for monomeric captopril particularly after administration of the dimer. Both the dimer and its pharmacologically active product captopril were found in relatively large amounts in lung, kidney and liver following the oral administration of the dimer.