N-Nitrosonornicotine (NNN) is one of the most abundant strong carcinogens in unburned tobacco and cigarette smoke and is classified by the International Agency for Research on Cancer as carcinogenic to humans. Human exposure to NNN mainly occurs upon use of tobacco products. It is also possible that additional amounts of NNN are formed endogenously. The goal of this study was to evaluate the inhibitory effect of some antioxidants, including ascorbic acid and grape seed extract (GSE), on endogenous NNN formation in rats treated with nornicotine and sodium nitrite by gavage twice daily for 3 days. The study included four groups of rats: (1) negative control group A, to which no chemical was administered; (2) negative control group B, treated with nornicotine alone (2.5 μmol per gavage); (3) positive control group, to which both nornicotine (2.5 μmol per gavage) and sodium nitrite (7.5 μmol per gavage) were administered; and (4) rats treated with nornicotine (2.5 μmol per gavage), inhibitor (7.5 or 37.5 μmol per gavage), and sodium nitrite (7.5 μmol per gavage). The mean (±SD) total amount of NNN in the 3-day urine of rats treated with both nornicotine and sodium nitrite was 4.78 ± 2.88 nmol. The order of inhibition of endogenous NNN formation in rats at the molar ratio [nitrite]:[inhibitor] 1:5 was as follows: ascorbic acid (91%) > dihydroxyfumaric acid (86%) ≈ catechin (85%) > resveratrol (no inhibition). Treatment of rats with grape seed extract did not produce statistically significant inhibition of endogenous nornicotine nitrosation. This is the first study that demonstrates endogenous NNN formation in rats treated with nornicotine and sodium nitrite and effective inhibition of this process by ascorbic acid, dixydroxyfumaric acid, and catechin.