A swim tunnel respirometer and an extracorporeal blood circulation technique allowed continuous collection of data from exercising fish below and near their critical speed. Swimming at speeds below maximum showed no changes in plasma Na+, Cl- and lactate concentrations but increased levels of blood hemoglobin and plasma K+. PaO2 and PaCO2 showed an exponential decrease and increase respectively and related to swimming speed. Increased swimming speed changed the acid-base status toward a mixed respiratory and metabolic acidosis. Upon reaching maximum speed sudden and large increases in plasma Na+, K+ and lactate concentrations occurred associated with a large metabolic acidosis.