Antitumor activity and lethality of cyclophosphamide alone and in combination with several drugs were investigated in male ddY mice. The antitumor activity was estimated by weighing the solid tumor on the 15th day after Ehrlich ascites cell inoculation. Pentobarbital induced sleeping time for monitoring the activity of hepatic drug-metabolizing enzymes was defined as the time between the loss and the recovery of the righting reflex. Consecutive administration of pentobarbital shortened the pentobarbital sleeping time and increased the antitumor activity after cyclophosphamide. On the contrary, a single administration of SKF 525A or cycloheximide prolonged the pentobarbital sleeping time significantly and decreased the antitumor activity after cyclophosphamide. Consecutive administration of aminopyrine, or chlorpromazine shortened the pentobarbital sleeping time and increased the antitumor activity after cyclophosphamide. These results indicate that aminopyrine and chlorpromazine may increase the levels of the hepatic drug-metabolizing components and may activate cyclophosphamide by conversion to an active form. Effect of a consecutive administration of morphine on the pentobarbital sleeping time and the antitumor activity was uncertain in individual cases. On the other hand, aminopyrine, chlorpromazine, or morphine in consecutive administration increased the lethality of cyclophosphamide.