The effects of nitrite-induced methaemoglobinaemia on adrenergic proton extrusion from rainbow trout red blood cells were studied using the pH-stat method. In control conditions adrenergic proton extrusion was completely inhibited by amiloride and was greater in deoxygenated than in oxygenated erythrocytes. Nitrite-induced methaemoglobinaemia was associated with a pronounced reduction in the catecholamine-stimulated proton efflux from both deoxygenated and oxygenated erythrocytes. In deoxygenated erythrocytes the initial proton efflux upon catecholamine stimulation decreased by 60-70%, while the percentage of methaemoglobin in the red cells increased from the control level of 1-3% to 20%. In oxygenated erythrocytes the decrease was 30% at the same methaemoglobin percentage range. It is suggested that the pronounced influence of nitrite-induced methaemoglobinaemia on adrenergic proton efflux results from an inhibition of the red cell sodium/proton exchanger by the R-like haemoglobin conformations.