Hematopoietic stem cell transplantation patients conditioned with cyclophosphamide (CY) and total body irradiation have substantially greater risk of nonrelapse mortality when plasma area under the concentration-time curve (AUC) of O-carboxyethylcyclophosphoramide mustard (CEPM) is high. The discovery was paradoxical because CEPM is a nontoxic elimination route of the protoxic CY metabolite hydroxycyclophosphamide (HCY). CY was administered to Wistar and TR- rats (a Wistar strain lacking functional ABCC2) at doses of 100 and 200 mg/kg CY, respectively. After either dose, Wistar rats excreted 4-glutathionylcyclophosphamide (GSCY) abundantly in bile; GSCY was absent from bile of TR- rats. Liver AUC(GSCY) was 2- to 2.5-fold greater in TR- than Wistar rats after 100 and 200 mg/kg CY, respectively. Liver AUC(HCY) was 24-46% greater in TR- rats than in Wistar rats after the respective CY doses. Plasma AUC(CEPM) of TR- rats was approximately twice that of Wistar rats after 100 mg/kg, but did not differ between the two strains after 200 mg/kg. Conversely, plasma AUC(HCY) was not different after 100 mg/kg CY, but was 40% greater in TR- rats after 200 mg/kg. The dose dependence of plasma AUC(CEPM) and AUC(HCY) was explained by the concentrations of HCY attained and the in vitro K(m) of aldehyde dehydrogenase and inhibition of aldehyde dehydrogense in TR- rats. We conclude that GSCY is a substrate of ABCC2, and plasma AUC(CEPM) functions as a reporter of liver exposure to HCY and toxins formed from it when HCY concentration is below the K(m) of aldehyde dehydrogenase and the activity is not compromised.