Exhaustive exercise causes PaCO2 elevation, intense acidosis, and a mobilization of catecholamines into the blood of trout. A radioisotopic assay (Wood and Perry, J. Exp. Biol. 157: 349-366, 1991) was used to examine changes in HCO3- dehydration rate in blood sampled by dorsal aortic catheter from exercised trout. The rate increased about 50% during and throughout the 30 min period following exercise when the blood was assayed immediately at the acid-base status prevailing in vivo. However when the blood from exercised fish was rapidly equilibrated to resting levels of PCO2 prior to assay, the rate was decreased by 15-35% relative to resting values. This effect was seen only in whole blood, not in plasma, indicating an inhibition at the level of the red cells. Conversely, when blood from resting trout was equilibrated to the elevated PaCO2 seen in vivo after exercise, the measured dehydration rate was greater than twice that in samples from exercised fish. Thus a relative inhibition of HCO3- dehydration through the red cells occurs after exercise, an effect likely associated with adrenergic activation of Na+/H+ exchange.