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1-Ethyl-2-benzimidazolone (EBIO) caused a sustained increase in electrogenic Cl⁻ secretion in isolated mouse colon mucosae, an effect reduced by blocking basolateral K⁺ channels. The Ca²⁺-sensitive K⁺ channel blocker charybdotoxin (ChTX) and the cAMP-sensitive K⁺channel blocker 293B were more effective when the other had been added first, suggesting that both types of K⁺ channel were activated. EBIO did not cause Cl⁻ secretion in cystic fibrosis (CF) colonic epithelia. In apically permeabilized colonic mucosae, EBIO increased the K⁺ current when a concentration gradient was imposed, an effect that was completely sensitive to ChTX. No current sensitive totrans-6-cyano-4-(N-ethylsulfonyl-N-methylamino)-3-hydroxy-2,2-dimethylchromane (293B) was found in this condition. However, the presence of basolateral cAMP-sensitive K⁺channels was demonstrated by the development of a 293B-sensitive K …