A convenient synthesis of pyrido[4,3-d]pyrimidine-2,4(1H,3H)-diones is described. Treatment of 5-formyl-1,3-dimethyl-6-(2-dimethylamino)-vinyluracil (1) with ammonia and hydrazines affords the corresponding pyrido[4,3-d]pyrimidine-2,4(1H,3H)-diones (2 and 3), respectively. Similar reaction of 5-cyano- and 5-ethoxycarbonyluracils (10 and 15) with ammonia led to formation of 5-amino- and 5-hydroxylpyrido[4,3-d]pyrimidine-2,4(1H,3H)-diones (11 and 16), respectively.