Electrogenic ion transport in the isolated co-Ionic epithelium from normal and transgenic mice with cystic fibrosis (CF mice) has been investigated under short-circuit current (I _sc) conditions. Normal tissues showed chloride secretion in response to carbachol or forskolin, which was sensitive to the Na-K-2Cl cotransport inhibitor, frusemide. Responses to both agents were maintained for at least 12 h in vitro, but the responses to carbachol changed in format throughout this period. By contrast CF colons failed to show the normal secretory responses to carbachol and forskolin, most preparations showing a decrease in I _sc that was immediately reversed by frusemide. In CF colons addition of Ba²⁺ ions or tetraethylammonium (TEA⁺) to the apical bathing solution antagonised the reduction in I _sc caused by the secretagogues. It is concluded that the reduction in I _sc in CF colons is due to electrogenic K⁺ secretion and this was confirmed by flux studies using rubidium-86. In normal colons exposed to TEA⁺ the responses to for-skolin were greater, but not significantly so, presumably because the minor K⁺-secretory responses are dominated by major chloride-secretory responses. Again rubidium-86 fluxes showed an increase of K⁺ secretion in normal colons receiving forskolin. Since the amiloride-sensitive current was not different in CF and normal colons there was no evidence that the CF mice were stressed in a way that increased mineralocorticoid levels and hence K⁺ secretion. Knowledge of the phenotype of the colonic epithelium of the CF mouse sets the baseline from which attempts at gene therapy for the gut must be judged.