Salivary nitrate and nitrate concentrations, in vitro kinetics of nitrate reduction in saliva, and numbers of salivary nitrate-reducing micro-organisms were each compared with N-nitrosoamino acid excretion in 16 humans eating controlled diets. N-Nitrosoproline (NPRO) and N-nitrosothiazolidine-4-carboxylic acid (NTCA) excretion were measured after intake of nitrate (5.24 mmol) and L-proline (4.35 mmol) before and after treatment with an oral antiseptic (Peridex, 0.12% chlorhexidine gluconate). Peridex treatment resulted in a 94% reduction in the numbers of salivary nitrate-reducing bacteria, a decline from 17 to 4% in the amount of salivary nitrate that was reduced to nitrite in vivo, and an 85% reduction in the rate of in vitro nitrate reduction by saliva. Concurrently, there were 62 and 74% inhibitions of endogenous NPRO and NTCA formation, respectively. Correlations of the numbers of nitrate reductase micro-organisms, in vivo oral nitrate reduction and salivary nitrite concentrations, with individual NPRO excretion, indicated that individuals with higher oral nitrate-reducing capacities formed more N-nitrosoamino acid endogenously. These data suggest that individual differences in oral nitrate reduction are a significant factor in gastric nitrosation and account for a large proportion of the interindividual variability in nitrosoamino acid excretion.