Various 6-[[(aryl and aralkyl)amino]methyl]-2,4-pteridinediamines and their 8-oxides have been synthesized for antimalarial evaluation. Condensation of 3-amino-6-(bromomethyl)-2-pyrazinecarbonitrile 4-oxide (V) with the appropriately substituted amine afforded a series of 3-amino-6-[[(aryl and aralkyl)amino]methyl]-2-pyrazinecarbonitrile 4-oxides VI. Deoxygenation gave the corresponding pyrazines VII. Cyclization of VI and VII with guanidine then produced the desired 6-(aminomethyl)-2,4-pteridinediamine N-oxides VIII and teridinediamines IX, respectively. Formylation of 6-[[(3,4-dichlorophenyl)amino]methyl]-2,4-pteridinediamine gave N-[(2,4-diamino-6-pteridinyl)-methyl]-N-(3,4-dichlorophenyl)formamide. The N-oxides VIII did not exhibit significant activity against Plasmodium berghei infections in mice. Activity among the 2,4-pteridinediamines IX was generally poor with the exception of the 3,4,5-trimethoxyphenyl and 1-naphthalenyl analogues which showed strong suppressive activity at doses ranging from 80 to 640 mg/kg. Furthermore, several of the 2,4-pteridinediamines exhibited potent prophylactic activity against Plasmodium gallinaceum infections in the chick and also showed strong antibacterial action against Streptococcus faecalis and Staphylococcus aureus.