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1. The short-term uptake of Na by the goldfish mucosa was compared using both inulin and choline as markers of extracellular space. The results were virtually identical, the distribution of both choline and inulin increasing rapidly to measure a space at 1 min which then remained nearly constant during a following 4 min incubation.

2. Using inulin as space marker, the uptake of various alkali metal cations was determined from a 1 min contact with the mucosa. The relative rates of uptake were Tl > K > Rb > Cs > Na > Li, with a low selectivity ratio, the range of permeabilities being no greater than 5.

3. The selectivity sequence was the same in both salt and fresh-water adapted fish. Of the alkali metal cations tested, only Na showed a significantly decreased uptake on adaptation to salt.

4. Isolated intestinal preparations from salt-adapted fish showed a reduced short-circuit current compared with fish adapted to fresh water, the values being 12·3 ± 1·2 and 35·7 ± 1·5 µA cm^-2 respectively. In both cases the short-circuit current was equivalent to the net transport of Na measured isotopically.

5. In Krebs—Henseleit medium, the measured tissue resistance was approximately 100 cm^-2 for both salt and fresh-water adapted fish.

6. It is concluded that regulation of cation transport in goldfish intestinal epithelium is specific for Na and mediated primarily through cellular rather than extracellular pathways.