The aim of the present study was the characterization of the subtypes of inositol 1,4,5-trisphosphate receptors (IP 3 R) in rat colonic epithelium. A monoclonal antibody against IP 3 R1 did not stain the colonic epithelial cells. In contrast, IP 3 R2 and IP 3 R3 were found within the epithelium; however, with a distinct intracellular localization and differences in their distribution along the crypt axis. IP 3 R2 immunoreactivity was found within the nuclei of the epithelial cells. The signal was distributed all over the nucleus and not restricted to the nuclear envelope as demonstrated by counterstaining with lamin B1 and electron microscopical examination after immunogold labelling. In contrast, an antibody against IP 3 R3 stained the epithelial cells mostly in their apical half in accordance with the typical localization of IP 3 R in organelles such as the endoplasmic reticulum. In addition, there was a gradient from the surface region towards the crypt fundus, where the IP 3 R3 signal could not be detected.Despite the strong IP 3 R3-gradient, in saponin-permeabilized colonic crypts exogenously administered IP 3 or adenophostin A evoked a similar depletion of mag-fura-2-loaded intracellular Ca 2+ stores in crypt and surface cells suggesting a contribution of the nuclear IP 3 R2 to the Ca 2+ release. This conclusion was confirmed by experiments with isolated nuclei from colonic epithelium, at which IP 3 was able to induce changes in the Ca 2+ concentration, which were inhibited by 2-aminoethoxy-diphenylborate (2-APB), a blocker of IP 3 receptors. These results demonstrate that the colonic epithelial cells undergo changes in IP 3 R subtype expression during differentiation.