N-Nitroso compounds are known to form endogenously in the human stomach from the precursors nitrite as well as secondary and tertiary amines. Ascorbic acid has been found to be an effective blocking agent for this process of formation of carcinogenic substances, both in vitro and in vivo. The mechanism by which ascorbic acid reacts is chemical in nature and results in the formation of NO and dehydroascorbic acid. Since NO can be recycled in the presence of O2 to form [NO]x, which is capable of additional nitrosation, greater than stoichrometric amounts of ascorbate are required for effective inhibition in vivo. The efficacy of ascorbic acid for inhibition of nitrosamine formation in humans has been demonstrated both clinically and epidemiologically through the use of "Nitrosoproline Test"--of Ohshima and Bartsch.