CYCLICAL transmission of P. berghei has been successfully carried out under experimental conditions. A batch of laboratory-bred A. quadrimaculatus and A. stephensi fed on February 15 on a hamster infected eight days previously with the Kasapa strain of P. berghei. The hamster showed numerous gametocytes in the peripheral blood at the time of the blood meal. Dissections of mosquitoes' stomachs showed a 50 per cent infection-rate. On February 25, twenty A. stephensi and twenty-two A. quadrimaculatus of this batch bit and fully engorged themselves on a month-old white rat. The salivary glands of these mosquitoes were dissected out in Ringer-glucose and injected into the same animal on February 26.