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The involvement of the gill and kidney in acid-base regulation was examined in the American eel, Anguilla rostrata, during 36h of continuous air-exposure and subsequent return to water. While in air, eels developed a severe mixed respiratory/- metabolic acidosis. Renal acid excretion increased only slightly during the latter stages of air-exposure. A pronounced reduction in urine flow rate was important to minimize dehydration but essentially eliminated the kidney as a route for excess acid excretion. Upon return to the water, eels had accrued an extracellular metabolic acid load of 9.53 mmol 1−1. The metabolic acid was cleared from the extracellular compartment at an exceptionally low rate (approximately 70μmol kg−1 h−1) and about 50 % of the acid load remained after 18 h of recovery in water. The clearance of metabolic acid was accounted for by enhanced branchial acid excretion which was related primarily to adjustments of unidirectional Na+ fluxes. Unidirectional Cl− fluxes were undetectable using radiotracer methods. We speculate that the inefficiency of acid-base regulation in the eel compared to other teleosts is, in part, related to the absence of significant branchial C1−/HCO3− exchange.