Four experiments designed to test the efficiency of monthly suppressive doses of antimalarial drugs for the control of malaria in indigenous West Africans are reported. The results consistently showed that monthly pyrimethamine in 25 mgm. doses to adults and children over three years and 12.5 mgm. doses to children under three years, in the absence of mosquito control measures, was effective in markedly lowering parasite rates and reducing clinical malaria attacks for periods of from one to two years. They further demonstrated that if strains of P. falciparum resistant to pyrimethamine occur in this part of West Africa, they are much less prevalent than reports from East Africa would indicate for that area and do not interfere with malaria control under the conditions of the experiments described in this report.