Abstract: The mice were administered ig or im with 100mg base/kg pyronaridine 15 days before (D-15) ip inoculation (D0) of Plasmodium yoelii-infected erythrocytes, and parasitemia did not occur in most of the drugged mice on D6 and D10. Similar results were obtained, with 10mg base/kg of pyronaridine given ig or im on D_10, 5mg/kg im on D_6 and 5mg/kg ig on D_4. Nevertheless, a dose of 400mg base/kg of chloroquine given either ig or im on D_3 failed to protect mice from P. yoelii infection, since the P. yoelii strain 265 used in the experiment had a low level of innate resistance to chloroquine.It was revealed in the expertments with P. berghei that the duration of schizonto-cidal effect of pyronaridine was markedly longer than that of chloroquine. A. dose of 10mg/kg pyronaridine ( 1.47 times ED50, or 0.7% LD50) given on D_6 exerted apparently suppressive action on blood schizonts, while an equivalent dose of 67mg/kg chloroquine (1.47 times ED50, or 10% LD50) showed slightly suppressive activity only when the drug was administered on D-2.