2, 6, 8-Trimethyl-, 6, 8-dimethyl-2-phenyl-, and 2-phenyl-pyrido [3, 4-d][1, 3] oxazin-4-ones have been prepared from the corresponding 3-aminopyridine-4-carboxylic acids. Treatment of the pyrido-oxazines with amines yielded the corresponding pyrido [3, 4-d] pyrimidines or intermediate 3-aminopyridine-4-carboxamides. Hydrazinolyses and methylations of a number of pyrido [3, 4-d] pyrimidin-4 (3H)-ones and-pyrimidine-2, 4 (1H, 3H)-diones are ...