Strains of Aedes aegypti were infected with the 8A strain of Plasmodium gallinaceum. 72 hrs. later the mosquitoes were fed antimalarial drugs. Quinine, quinacrine, plasmochin, and SN6911 in maximum tolerated doses did not affect sporozoite production or viability. In infected mosquitoes treated with Na sulfadiazine, oocysts failed to develop properly and sporozoites were produced only rarely. Suspensions of mosquitoes prepd. from those maintained on sugar solns. and on quinine, quinacrine, plasmochin, and SN6911 contain 2-3 sporozoites per oil immersion field, whereas it may require 10 min. to find any sporozoites in suspensions maintained on 0.1% Na sulfadiazine. Inoculation of mosquitoes maintained on sulfadiazine levels above 0.1% has failed to produce infection. Reinoculation of these negative chicks with P. gallinaceum caused characteristic infections with high parasitemias. At a 0.1% level oocysts formed in large numbers but rarely grew beyond the medial point of development. With sulfadiazine conc. of 0.3%, oocysts were more completely inhibited. Below 0.1%, sporozoite production and viability were not very seriously affected. There seems to be a definite relation between drug activity and effect in the sporozoite-infected vertebrate host and drug activity in the infected invertebrate host.