The effect of unisexual schistosome infection on the activities of several hepatic enzymes was studied in mice. The activities of hepatic drug-metabolizing enzymes in mice infected with either female or male schistosomes were not significantly different from those of noninfected control animals. However, the total amount of heme pigment in the liver of infected mice was 2.7 (female infection) and 8.9 (male infection) times greater than that of control animals. The durations of hexobarbital sleeping times and of zoxazolamine paralysis in unisexual schistosome infections did not differ from those of uninfected controls. Therefore, an accumulation of schistosome pigment without egg deposition, as in this unisexual infection study, does not result in a severe reduction of hepatic drug-metabolizing capacity.