A series of 6,7,8-substituted 3-methylpteridine-2,4(3H,8H)- diones was synthesized by condensation of 5-amino-6-(substituted anilino )-3-methylpyrimidine-2,4(1H,3H)- dione with α- dicarbonyl reagents. These compounds were tested for antimalarial activity against the human parasite Plasmodium falciparum in culture, and lethal Plasmodium vinckei vinckei in mice. The 6,7-unsubstituted pteridinediones were active against cultured P. Falciparum ; however, none of the series was active against P. vinckei vinckei in mice.