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1. The unidirectional fluxes of ²²Na, ³⁶Cl and [¹⁴C]bicarbonate ions were measured in paired portions of the isolated and short-circuited colonic mucosa of Bufo arenarum, separated from its muscular layer. Pharmacological effects as well as effects of changes in the composition of the nutrient solutions on the electrical parameters of membrane activity (potential difference, short-circuit current and total membrane resistance) are described.

2. The net fluxes of both Cl and bicarbonate ions were not significantly different from zero in the absence of electrochemical gradients across the membrane.

3. The net Na flux from mucosa to serosa represented a variable proportion of the short-circuit current ranging from 62 to 100%.

4. The proportion of membranes with high discrepancies between net Na flux and short-circuit current decreased with the duration of captivity of the toads.

5. When Na was entirely replaced by choline in the mucosal bathing solution, the short-circuit current dropped by a variable amount within the range of 64 to 98% of its control values in different membranes. This effect was completely reversible. Similar changes in the serosal solution had no effect.

6. The short-circuit current and potential difference were very sensitive to the serosal concentration of bicarbonate ions. In different membranes, 60-100% of the short-circuit current was reversibly abolished by bathing the serosal surface with a bicarbonate-free solution. The mucosal bicarbonate level had no effect on either the potential difference or the short-circuit current. 5 mM bicarbonate in the serosal solution restored at least 50% of the short-circuit control value and full recovery was attained by concentrations near 30 mM bicarbonate.

7. Anoxia brought the potential difference and short-circuit current reversibly down to zero in about 50 min.

8. Ouabain reduced the short-circuit current up to 80% in about 40 min when present in the serosal solution at a concentration of 10^-4 M. At this or lower concentrations the ouabain effect was reversible. Above this level ouabain produced 100% inhibition in 3-4 hr, but this was no longer reversible. Ouabain had no effect on the short-circuit current either when applied to the mucosal surface or in the absence of Na from the mucosal solution.

9. Diamox produced a variable inhibition of the short-circuit current of up to 30% only at concentrations above 10 mM.

10. Possible mechanisms are discussed for the appearance of the non-Na component of the short-circuit current. A theory concerning its nature is proposed.