A flow injection method for determination of nitrite and nitrate in freshwaters is described based on luminol-hypochlorite chemiluminescence (CL) system. Nitrate is reduced on-line with a cadmium reduction column to nitrite and its inhibition effect on luminol CL emission was measured. The effects of chemical and physical parameters such as buffer pH and concentration, luminol, sodium hypochlorite and sulfuric acid concentrations, flow rate, and sample volume were investigated. The calibration graphs were linear over the range 0.1-50 µM (R2 = 0.9989 and 0.9984) for nitrite and nitrate respectively with a limit of detection (S/N = 3) of 4.0 × 10-8 M and a sample throughput of 120 samples per hour. The effect of foreign ions was studied and the method was successfully applied to the determination of nitrite and nitrate in water samples. The results obtained were in good agreement with those achieved by a spectrophotometric reference method at the 95% confidence level. Standard addition method was also applied to the freshwater samples and the recovery values were found in the range of 92-109% and 94-105% for nitrite and nitrate respectively.