The synthesis and characterization of N-(N-acetylprolyl)-N-nitroso-glycine, the first authentic N-nitrosopeptide, is described, and its stability under various conditions is reported. In acidic media, denitrosation and deamination (hydrolysis) occur concurrently, whereas in neutral and alkaline solutions, only deamination occurs. The rates of formation and decomposition of some unprotected N-nitrosopeptides in strong acid are also reported. Conditions for the formation of such compounds in the gastric tract are discussed, together with their potential involvement in human cancer.