I. The effects of different external media on the sodium and chloride efflux in Artemia salina, the brine shrimp, have been observed, using animals acclimatized to sea water. In sea water, both sodium and chloride fluxes across the epithelium are approximately 7,000 pmole cm.-2 sec.-1.

2. Sodium efflux drops markedly in sodium-free media, and chloride efflux falls in chloride-free media; the two effects are independent, and are not due to changes in external osmolarity.

3. The decreases in sodium efflux can be explained by changes in electrical potential difference and diffusional permeability; exchange diffusion of sodium does not occur.

4. Approximately 70% of the chloride efflux is due to exchange diffusion, and most of the remainder is due to active transport.

5. It is shown that graphs of ion efflux against external concentration which can be fitted by a Michaelis-Menten equation do not constitute evidence for the presence of exchange diffusion; graphs of similar shape can be obtained if the flux is simply diffusional.

6. The drinking rate, determined from the rate of uptake of 131I-polyvinylpyr-rolidone, is 36 pl. sec.-1, or 2.0% body weight hr.-1.

7. The diffusional influx of water is 240 pl. sec.-1.

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