The toxicity of combined use of blood schizontocide pyronaridine (PND) and primaquine (PQ) in mice and rats was significantly lower than that of chloroquine (CQ) plus primaquine (PQ). PND 1/2 LD50 (ca 600 mg/kg) in combination with PQ reduced the toxic action of PQ in mice, while CQ 1/2 LD50 (ca 300 mg/kg) plus PQ produced synergistic toxic effect. In animal models such as Plasmodium yoelii sporozoite infection in mice and P. cynomolgi sporozoite infection in rhesus monkeys, the tissue schizontocidal action of PQ was not affected by PND. Therefore, clinical evaluation of PND/PQ in comparison with CQ/PQ in treating vivax malaria is suggested.