Abstract: When P. yoelii gametocyte-carrying mice administered ig with pyronaridine 1,200 mg base/kg ( LD50) were fed 2 or 12 hours later to batches of Anopheles stephensi, no affection was observed on the development of oocyst and infective sporo-zoite in the mosquitoes. Similar result was obtained in mosquitoes fed on mice medicated with chloroquine 600 mg base/kg ( LDSO). However, the formation of oocyst was inhibited when the mosquitoes were fed on mice drugged ig with 20 mg base/kg of primaquine or a larger dose of pyrimetltiamihe (10 mg/kg). When 0.005% pyronaridine was fed to the mosquitoes after their engorgement of gametocyte-blood meal, no influence on gametocyte infectivity and sporogony was noted. With 0.01% pyronar ridine most mosquitoes died of intoxication, but normal oocyst could still be found in the survivors. The result in mosquitoes exposed to ehloroquine was similar, normal oocysts being detected. In contrast, the feeding of 0.001% pyrimethamine to mosquitoes led to complete suppression of sporogony. 0.01% primaquine caused the death of many mosquitoes, but oocysts could still be found in the remainders. Primaquine inhibited the infectivity of gametocytes to mosquitoes and oocyst formation at the concentration of 0.1%, no oocyst being detected in the few survivors.