Subchronic toxicity of nitrite in rainbow trout (Oncorhynchus mykiss; mean mass ± S.D., 18.9 ± 1.3g) was assessed in a 28-day trial. The influence of nitrite on fish mortality, growth rate, haematology, blood biochemistry, and gill histology was observed. Survival was not affected by exposures up to 1 mg l(-1) NO(2)(-) (at 10 mg l(-1) Cl(-)). On the basis of growth rate inhibition data, the values of NOEC (28 d LC(0)) and LOEC (28 d LC(10)) were estimated at 0.01 and 0.2 mg l(-1) NO(2)(-), respectively. At 0.01 mg l(-1) NO(2)(-) (the lowest concentration tested), there was segmental hyperplasia of the respiratory epithelium of secondary lamellae and elevated glucose and decreased potassium. Elevated nitrite concentrations were found in blood plasma of fish exposed to concentrations of 1.0 mg l(-1) NO(2)(-) and higher, and in muscle tissue at the highest concentration 3.0 mg l(-1) NO(2)(-). Plasma and muscle nitrite levels were lower than those in the ambient water in all experimental groups.