A simple, sensitive and selective fluorimetric method for the determination of nitrite ion in waters using a merging zones flow injection system is described. The fluorimetric determination is based on the measurement of the quenching effect produced by nitrite on proflavine (3,6-diaminoacridine) fluorescence (lambda(ex)/lambda(em)=290/519nm). The optimum experimental conditions were investigated by merging 0.5ml of the sample and 0.5ml of a solution of 5mgl(-1) of proflavine (in 0.1M HCl) in a flow injection system, on-line connected to a flow-cell placed in the conventional sample compartment of a spectrofluorimeter. The selected carrier solution and final flow rate were 0.1M HCl and 0.5mlmin(-1), respectively. A reaction coil of 2ml was used. As a result of the simplicity of this system, a sample throughput of about 50 samplesh(-1) can be achieved with the proposed methodology. The detection limit was 1.1ngml(-1) (3sigma criterion) of nitrite. The repeatability for five sample injections containing 100ngml(-1) of nitrite was +/-0.3% and the observed linear range extended up to 400ngml(-1). Also, the effect of interferences from various metals and anions commonly present in waters was also studied. The method was successfully applied to the determination of low levels of nitrite in different water samples (river, fountain, tap and commercial drinking waters).