The mass spectra of the four pyrido[2,3-d]-, pyrido[3,2-d]-, pyrido[3,4-d]-, and pyrido[4,3-d]-pyrimidin-4(3H)-ones, and the corresponding four -pyrimidine-2,4(1H,3H)-diones, and a number of methyl-, hydroxy-, and phenyl-substituted derivatives of these compounds have been measured. Fragmentation pathways are postulated on the basis of these spectra and, in certain cases, with the aid of deuterium labelling. Variations are observed in the mode of fragmentation according to the nature of the substitutent group and the position of the nitrogen atom in the pyridine ring, and comparisons are drawn with the quinazolones and pteridones.