1. 1.

    Four species of marine teleosts were shown to possess an external-NH4-inhibited Na uptake from 1 mM-NaCl solutions. The inhibition was not due to changes in the transepithelial potential.

  2. 2.

    Injection of 2 μM-NH4/g fish stimulated Na uptake by Opsanus beta and also stimulated ammonia efflux, 50% of which was dependent upon external Na.

  3. 3.

    The ammonia efflux from three species was partially dependent upon external Na.

  4. 4.

    Na/NH4 exchange in O. beta could be reversed so that 22Na efflux could be stimulated by the addition of 200 mM-NH4 to the external solution.

  5. 5.

    These studies show clearly that marine teleosts possess an Na/NH4 exchange system in sea water which results in a net influx of Na into the fish.

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