A novel sequential injection method for the determination of nitrite at nanomolar level in seawater samples has been developed. The pink azo compound was formed based on the Griess reaction and quantitatively adsorbed onto a Sep-Pak C18 cartridge. The enriched azo compound was rinsed with water and ethanol (28%, v/v) in turn, and then eluted with an eluent containing 26.6% (v/v) ethanol and 0.108 mol L(-1) H(2)SO(4). Finally the azo compound was measured using a spectrophotometer at 543 nm. Under the optimized conditions, the linear calibration ranges were 0.71-42.9 nmol L(-1) for a 150-mL sample and 35.7-429 nmol L(-1) for a 15-mL sample. The relative standard deviation of 8 measurements was 1.44% for 14.3 nmol L(-1) nitrite. For the 150 mL sample, the detection limit was estimated to be 0.1 nmol L(-1). The throughput of the method was about 4 samples per hour. The proposed method has been successfully applied to the in-field determination of nanomolar concentrations of nitrite in seawater.