Big-conductance Ca(2+)-activated K(+) channels (BK channels) may play an important role in the regulation of epithelial salt and water transport, but little is known about the expression level and the precise localization of BK channels in epithelia. The aim of the present study was to quantify and localize the BK channels in the distal colon epithelium by iberiotoxin (IbTX) binding using the radiolabeled iberiotoxin analogue (125)I-IbTX-D19Y/Y36F, by autoradiography and by immunohistochemical studies. The results showed that the surface cells, responsible for Na(+) absorption, contained a high number of BK channels, whereas the abundance of the channels in the Cl(-)-secreting crypt cells was very low or absent. Surprisingly, the (125)I-IbTX-D19Y/Y36F binding and immunohistochemical studies showed expression of BK channels in the apical as well as in the basolateral membranes of the surface cells. In conclusion, the significant and distinct expression of BK channels in epithelia, combined with their strict regulation, indicate that these channels may play an important role in the overall regulation of salt and water transport.