1. Ascending and descending segments of colon of normal and Nadepleted rats were perfused with solutions of differing KCl concentration. Net K flux, electrical p.d. and, in some experiments, unidirectional K fluxes were measured.2. Variation of luminal K concentration over the range 0-40 mM did not affect p.d. or K efflux rate.3. K secretion rate fell about 30% when Na-free choline chloride solution was perfused.4. Net flux was a linear function of the luminal K concentration, and fell as the latter increased. Na depletion increased K secretion rate and passive permeability of the mucosa to K. Adrenalectomy had the reverse effect. The luminal K concentration, associated with zero net K flux was much greater than expected if the colonic mucosa behaved passively with respect to K.5. Unidirectional fluxes determined when 5 mM-KCl was in the lumen showed that the ratio influx/efflux was much less than predicted by Ussing's flux ratio equation.6. It was concluded that K influx was due to simple diffusion and K efflux to diffusion and active transport, both processes being increased by Na depletion.