Blood levels of cyclophosphamide (CP) and activated metabolites were measured in 11 patients undergoing a 2- to 4-day conditioning chemotherapy for bone marrow transplantation. Urinary excretion of CP was determined in five patients. CP half-life decreased after pretreatment from an average of 7.1 h on the 1st day to 5.5 h on the 2nd day (P less than 0.005) and to 4.3 h on the 4th day (P less than 0.005). No characteristic changes in urinary excretion could be observed. At the same time the exposure to non-protein-bound activated metabolites increased from 10.5 to 19.5 and 26.0 nmol x h/ml respectively (P less than 0.005 and P less than 0.04). Thus, in contrast to in vitro and animal studies, no evidence for an inhibition of activating enzymes could be found. On the contrary, pretreatment seems to enhance the production of the cytotoxic metabolites. The possible explanation of these changes by enzyme induction and by the role of saturated protein binding sites is discussed. Exposure to active metabolites might be altered by dose splitting or even by a change in the duration of the infusion.