1. 1.

    Rates of efflux of H+ and ammonia from a marine teleost and an elasmobranch were measured.

  2. 2.

    Hypercapnia stimulated H+ efflux from both species, stimulated ammonia efflux from the elasmobranch, and inhibited ammonia efflux from the teleost.

  3. 3.

    In both species the H+ and ammonia efflux were predominantly across the branchial epithelium. In Na+-free sea water, the H+ efflux from both species was completely abolished and the ammonia efflux was inhibited by approximately 50%.

  4. 4.

    Injection of an acid load stimulated H+ efflux, which continued for 2–5 h until more than the injected acid load was excreted. It therefore appears that injection of an acid load also produces a metabolic acid load which must be excreted.

  5. 5.

    The H+ efflux from mineral/metabolic acidotic fish is entirely branchial and dependent upon external Na.

  6. 6.

    The data support the conclusion that marine teleosts and elasmobranchs possess branchial Na+/NH4+ and Na+/H+ ionic exchange mechanisms and that Na+/H+ exchange plays a major role in the response to acidosis in both groups.

  7. 7.

    The possible evolution of these ionic exchange systems is discussed.

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