To evaluate the response of heme polymerase to treatment of malaria with chloroquine, we used mice infected with Plasmodium berghei. Six hours after treatment with 3 mumoles of chloroquine intraperitoneally per mouse, heme polymerase activity in parasitized erythrocytes decreased from 238 to 37 nanomoles of ferriprotoporphyrin IX polymerized per hour per mumole of ferriprotoporphyrin IX in preformed hemozoin, and nonhemozoin ferriprotoporphyrin IX increased in vivo from 40 to 123 nanomoles per ml of packed, parasitized erythrocytes. Other 4-aminoquinoline derivatives were similar in effect to chloroquine. Treatment with quinine, mefloquine, primaquine, or naphthalene derivatives caused no reduction in heme polymerase activity. In contrast to 4-aminoquinoline derivatives, quinine and mefloquine, which are quinolinemethanol derivatives, antagonized the effect of chloroquine.