Rainbow trout (30-70 g) were exposed to environmental nitrite for periods varying from 2 to 24 h, and after 12 h blood plasma nitrite concentration was eight times the environmental levels (0.5 mmol l-1). The rise was followed by an increase in methaemoglobin levels from around 3% to over 60%. After 2 h nitrite exposure the concentrations of plasma potassium, sodium and chloride fell, followed 2 h later by an increase in intra-erythrocyte potassium and sodium concentration with increased red cell volume. Twelve h nitrite exposure led to an increase in the red cell population, the new cells being smaller and containing less haemoglobin. After 24 h exposure, fish fell into two groups: nitrite-intolerant fish with high levels of plasma nitrite and methaemoglobin and nitrite-tolerant fish with low plasma levels. All fish surviving 24 h nitrite exposure had lower plasma potassium levels than unexposed fish.