Chloride interferes with nitrate determination in water by Indigo Carmine Method. The aim of this study was to eliminate chloride interference by masking it with excess chloride. 10.000 ppm chloride was found to be sufficient for this purpose. The modified method obeys Beer's law for 5.0-16.0 ppm NO3-N in samples and the calibration curve is A = - 0.0483 C + 1.1423 (r = -0.9985). The recovery of nitrate from spiked samples is 99.1%. Ions commonly encountered in drinking waters such as sulphate, phosphate, bicarbonate, ammonium, magnesium and calcium don't interfere up to 500 ppm. NO2-N interferes in 1 ppm concentration, but its concentration in drinking waters never exceeds 0.1 ppm. According to experimental data, the modified method can be used successfully as a limit test for nitrate in drinking waters.