A flow injection method for the determination of nitrate and nitrate in human saliva is described. It is based on the reaction of nitrite with iodide in acidic medium, the triiodide formed being amperometrically monitored at +0.2 V with a platinum microelectrode (r = 5μm) in a wall-jet cell. The method is very sensitive owing to the association of hydrodynamic transport in a flowing solution and the high mass transport rate at a microelectrode, the limit of detection for nitrite and nitrate being 0.07 and 0.25 μM respectively. The sample throughput was 60 h−1and the standard deviation for 5 injections of 1 μM nitrite was 1.9% (1.3% for nitrate). The method was used in the study of the conversion of nitrate to nitrite in the oral cavity by bacteria, experimental data concerning the salivary concentration of both ions being obtained before and after treatment with an antiseptic solution. The results showed the decline of nitrite concentration after the oral treatment followed by a proportional increase in the nitrate concentration, proving that the majority on salivary nitrite comes from bacterial reduction of nitrate.