We evaluated the ability of fresh tomatoes and processed tomato products (whole, diced, sauce, puree, and juice) to inhibit in vitro the formation of N-Nitrosomorpholine (NMOR), under conditions similar to the human stomach. The amount of NMOR that was formed averaged 23% to 82% that of the controls, on an equal wet weight basis, with paste being the most inhibitory. On an equal soluble solids basis, the amount of NMOR formed in the presence of products averaged 82% to 88% of that formed in the distilled water control. Fresh tomatoes showed greatest inhibition of nitrosation (NMOR formation averaged 80% of control), and processed tomatoes showed similar inhibition (NMOR 82% to 88% of the control). Ascorbic acid content was the strongest predictor of ability to inhibit NMOR formation. Processed tomato products inhibited nitrosation to a similar extent as had been reported for fresh tomatoes.