The colonic crypt is a bi-functional epithelial tissue. The ion and water absorption primarily occurs in the surface cells, whereas crypt cells are responsible for secretion. The ion transport in distal colon is regulated by aldosterone, which stimulates both Na⁺ absorption and K⁺ secretion. The electrogenic Na⁺ absorption is specifically mediated by ENaC channels in surface cells. Previously, we identified the big conductance Ca²⁺-activated K⁺ channel, K_Ca1.1, as the only functionally relevant K⁺ secretory pathway in mouse distal colon. The exact localization of K_Ca1.1 channels along the crypt axis, however, remains controversial. The aim of this project was to determine the precise localization of the K_Ca1.1 channel in the colonic epithelium. Here, we quantify K_Ca1.1 mRNA in micro-dissected isolated surface and crypt cells, using NKCC1 as a marker for crypt and g-ENaC for surface cells. The amount of mRNA was expressed relative to b-actin. We were able to verify that NKCC1 was expressed primarily in the crypts and g-ENaC primarily in the surface cells. The K_Ca1.1 a-subunit mRNA was like NKCC1, primarily expressed in the crypts. This finding substantiates our previous immunohistochemistry showing the K_Ca1.1 a-subunit mainly localize to the crypts. The overall expression pattern of the channels and transporters was not altered by aldosterone. The mRNA levels for NKCC1, g-ENaC and K_Ca1.1 a-subunit was, however, substantially augmented (K_Ca1.1 a-subunit-2 fold, NKCC1-2 fold and ENaC–10 fold) when the plasma aldosterone was elevated by increasing the dietary K⁺ content. All together, these findings suggest that the K_Ca1.1-mediated K⁺ secretion mainly is a feature of the colonic crypt in agreement with the general feature of secretory processes located to the crypt enterocytes.