The weak antiallergic activity of 6-methyl-4-oxo-6,7,8,9-tetrahydro-4H-pyrido[1,2-a]pyrimidine-3-carboxylic acid (4) on the rat reaginic passive cutaneous anaphylaxis test was enhanced by the introduction of appropriate functional groups into position 9 of the pyridopyrimidine ring. The most active 9-substituted pyridopyrimidinecarboxylic acids contained an oxime, a phenylamino, or a (phenylamino)thioxomethyl