1. The kinetics of chloride transport by the irrigated trout gill have been studied. The transport system is saturable, and the half-saturation value is about 0.25mM.

2. Chloride uptake occurs equally well from solutions of non-penetrating cations and from NaCl solutions. The presence of potassium in the irrigating solution, however, significantly inhibits chloride uptake.

3. The trout gill is permeable to potassium, and there appears to be an active component to potassium influx.

4. Chloride transport is stimulated by injections of NaHCO3 and (NH4)HCO3. The carbonic anhydrase inhibitor, acetazolamide, has no apparent effect on chloride influx, and it is suggested that if a Cl-/HCO3- exchange exists in the trout gill, sufficient HCO3- can be supplied by the blood.

5. The mean intracellular potential (relative to the irrigating solution) of five gill filament cells was -32 mV. This indicates the presence of an energy barrier to chloride uptake at the outer membrane of the epithelium, and therefore it is postulated that an active step for chloride transport is located on this membrane.

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