1. The metabolism of dexrazoxane (ICRF-187) and its optical isomer levrazoxane (ICRF-186) by the isolated rat hepatocyte was studied by hplc. 2. 4-Chlorobenzenesulphonamide, which is a strong inhibitor of dihydropyrimidine amidohydrolase (DHPase), caused 82% inhibition of the loss of dexrazoxane from the hepatocyte suspension. 3. Dexrazoxane was metabolized at an initial rate by isolated hepatocytes that was 1.8 times faster than levrazoxane. This ratio is close to that found for purified DHPase, suggesting that DHPase present in the hepatocyte catalyses the ring-opening hydrolysis of these drugs. 4. The ratios of the rates at which each of the one-ring open intermediates of dexrazoxane and levrazoxane were produced in the hepatocyte suspension are also consistent with DHPase being primarily responsible for the metabolism of dexrazoxane and levrazoxane. 5. Thus, the DHPase-catalysed formation of the one-ring opened intermediates enhances the rate at which the presumably active metal-ion binding forms of dexrazoxane are produced in the hepatocyte. 6. The DHPase content of the hepatocyte was estimated to be 1.2 nmol/kg of total hepatocyte mass, or equivalently 5700 molecules of DHPase per hepatocyte.