The Authors studied the response of human PLASMODIUM "in vivo" to antimalarial drugs in São Luís, Maranhão State, Brazil, during the years of 1981 and 1982. In the treatment of Plasmodium falciparum malaria was shown that 25.9% presented resistance to chloroquine and 16% to the association sulfonamide and pyrimethamine. With Plasmodium vivax it was found that effectiveness was 94.4% and 96% for both regimens of treatment. Finally the Authors suggest the use of the association chloroquine, primaquine and pyrimetamine in a three days scheme for better and low cost treatment. Also the associations sulfonamide, pyrimetamine and quinine as an alternative treatment for Plasmodium falciparum resistant to 4-amino-quinolines.