Abstract: The Plasmodium berghei parent line which had lost the ability of producing gameto-cytes was maintained soly by syringe passage for more than 50 years. After transmitting the parent line parasites under pyronaridine pressure for 114-134 syringe passages, a pyronaridine-resistant(RP) line at a high level 0300-fold) emerged. When the RP line Passage 114 and Passage 134 were further transmitted through 15-21 syringe passages in the absence of drug pressure, gametocytes abruptly reappeared in their blood. These reappeared gametocytes were fertile and the sporozoites developed in A. stephensi were infective to mice. The P. berghei RP line parasites through cyclic transmission retained their resistance to pyronaridine at a high level and cross-resisted highly to chloroquine.