6-Nitro[1,2,4]triazolo- and 6-nitropyrazolo[1,5-a]pyrimidines 1 a-h react with malononitrile and ethyl cyanoacetate to yield 9-imino-, 2a-h, and 9-oxo-7-nitro-4,9-dihydroazolo[1,5-a]pyrido-[2,3-d]pyrimidines 4a-g, respectively. By using the N∗C∗-double-labelled ethyl cyanoacetate in this reaction, its participation as a 1,3-bifunctional reagent was established. Malononitrile or ethyl cyanoacetate provide the C-C-N fragment for the pyridine ring. The mechanism is discussed.