A flow-injection manifold is proposed for determination of nitrite based on the reaction with 3,6-diamino acridine (proflavin sulfate) in hydrochloride acid medium. The assembly is adapted for nitrate determination by including a reductive column filled with copperined cadmium. The influence of foreign substances is also studied. The method gives a linear calibration graph over the range 0.06-4 mg I-’ nitrite, with an RSD ~0.5%. The method was applied to nitrite and nitrate determinations in either waste water or coastal marine water samples