The purpose of this investigation was to clarify the relation between the therapeutic efficacy of cyclophosphamide and the activity of the endogenous and exogenous corticosteroid hormones using ICR female mice with ascitic Ehrlich tumor. A single ip injection of the drug, 3 mg/mouse, was given to each mouse 36 hr after tumor inoculation (1 X 10(6) tumor cells, ip), and the drug effect was assessed in terms of the number of 1-month survivors with and without tumor. The clock time of drug administration which produced minimal therapeutic efficacy chronologically coincided with the dark period (8 PM approximately 8 AM) and/or with a stage of increased plasma corticosterone in mice. In support of this, concomitant administration of corticosterone (hydrocortisone or corticosterone) significantly depressed the anti-tumor effect of the drug. Evidence is presented to indicate that the adverse effect of corticosteroids is related to an accelerated metabolism of cyclophosphamide within the host. The drug effect for a given total dose was improved by the use of hourly partitioned injections in place of a single injection. It was concluded that the therapeutic efficacy of cyclophosphamide is dependent on the time of exposure to drug action on the scale of hours, but not days, and that the anti-tumor effect of the drug can be suppressed by increased activity of endogenous and exogenous corticosteroid hormones through acceleration of the drug metabolism.