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1. The transport of K across the mucosal epithelium of the colon of normal and Na-depleted rats has been investigated in vivo by methods involving I.V. administration of solutions containing ⁴²K and by labelling the mucosa with ⁴²K from the lumen and observing the `soaking-out' rates into radionucleide-free solutions.

2. During the infusions, the specific activity of the mucosal K was always less than the simultaneous specific activity of the secreted K. On stopping the infusion the specific activity of the mucosal K fell although the plasma specific activity remained at all times greater than that of the mucosa. The blood flow rate to the mucosa estimated from these results was 0·95 ml./min.g wet wt.

3. Experiments with ⁴²K in the lumen showed that the steady-state specific activity of the mucosal K was considerably less than expected if the K had been in a single compartment. The ⁴²K efflux curves were also consistent in showing two components. The curves were analysed on the basis of a model having two compartments in the mucosa with half-times of 6 and 33 min.

4. Na depletion produced two significant changes, (a) K secretion rate doubled, (b) the steady-state specific activity of the mucosal K after luminal exposure to ⁴²K was increased. The half-times of the compartments were unaffected.

5. The results were discussed in terms of a working hypothesis which represented the mucosal K in two compartments. Most of the K exchanges across the mucosa took place through the smaller compartment which probably contained 5% or less of the mucosal K, but which had a rapid turnover rate. The change of K permeability of the luminal face produced by Na depletion and probably due to increased aldosterone action, may well be an important factor in producing the rise of K secretion rate.