A fast and simple flow-injection method for sensitive automated determination of nitrates in various water samples is described. The method is based on the on-line photolytic conversion of nitrate in a quartz capillary to nitrite, which is determined by a chemiluminescent reaction. The photolytic conversion provides ecological detection of nitrate as compared with nitrate reduction at copperized cadmium. The detection limit is 2 10^-7 M nitrate, the calibration curve is linear to 1 10^-5 mol.l^-1. The efficiency of conversion of nitrate to nitrite is 63 %. The method is usable for the simultaneous determination of nitrate and nitrite in water.