The considerable differences in salivary nitrite levels observed previously in subjects who had abstained overnight from dietary nitrate as completely as possible were again found in tests on ten separate days. A 25-fold variation was observed in the rate of production of nitrite in vitro during the incubation of salivas from different subjects with 16 mM-nitrate under standardized conditions. N-Nitroso compounds were detected as a group on 11/100 and 14/100 occasions after incubations in vitro without and with added nitrate, respectively. No significant changes in salivary nitrite level were found 1 hr after the volunteers had ingested water-borne nitrate at approximately the World Health Organization recommended limit for continuous use (50 mg NO-3/litre), but increases in nitrite concentration were consistently found after volunteers ingested nitrate in water at four times this concentration. Although the nitrite concentrations were markedly increased following the intake of nitrate at the higher level, the occurrence of N-nitroso compounds in the salivas of the volunteers was greater immediately prior to the ingestion of water-borne nitrate than 1 hr afterwards.