Background: The vitamin D receptor (VDR) is a nuclear receptor that is expressed in many tissues throughout the body, but is particularly abundant within the colon. VDR is activated by the active form of vitamin D, calcitriol, but is also known as a receptor for the secondary bile acid, lithocholic acid. VDR has previously been shown to be anti-inflammatory and barrier promoting in the colon. However, the role VDR plays in regulating colonic epithelial transport function is less known. Interestingly, previous studies from our laboratory have shown the bile acid receptor, farnesoid X receptor (FXR), to exert anti-secretory actions in colonic epithelial cells. The current study aims to investigate a role for VDR in regulating colonic Cl- secretion and whether it may be a target for the treatment of diarrhoeal diseases. Aim: To investigate the effects of VDR activation by calcitriol on colonic epithelial Cl- secretion. Methods: Polarised monolayers of T84 cells were used as a model of the colonic epithelium. Protein expression of VDR was measured by western blotting. Calcitriol (1 – 20 nM) activated VDR and the VDR target protein, CYP24A1, verified activation of the receptor by qRT-PCR. Expression of the FXR target protein, FGF19, and FXR activation measured by luciferase activity verified the specificity of calcitriol to VDR. The effects of VDR activation by calcitriol (1 – 100 nM) on chloride secretion was determined using the Ussing chamber technique. Changes in short-circuit current (Isc) induced by the cAMP-dependent agonist, forskolin (FSK; 10 µM) or the Ca2+-dependent agonist, carbachol (CCh; 100 µM) were assessed. Results:  VDR was expressed in T84 cell monolayers as determined by western blotting. Treatment of T84 cells with calcitriol (1 – 20 nM) significantly (n = 3, *p<0.05) increased expression of CYP24A1, indicating activation of VDR. Calcitriol did not alter the activity of FXR, either in an FXR/luciferase reporter cell line or in T84 cells, indicating specificity of the agonist for VDR. LCA (10 µM) induced activation of both VDR and FXR. In Ussing chambers, the FXR agonist, GW4064, inhibited Cl- secretory responses to both FSK and CCh, as previously reported (n = 7, ***p< 0.001). Calcitriol (10 – 100 nM) was also found to significantly inhibit FSK-stimulated Cl- secretory responses by 37.9 ± 4.7 (p< 0.01) and 32.6 ± 7.2 (p< 0.05), respectively (n = 7), while 100 nM calcitriol significantly reduced CCh-induced responses by 31.4 ± 6.5 (n = 7, *p≤ 0.05). Analysis of the time course over which calcitriol exerts its effects revealed its antisecretory actions to be apparent after 24 – 48hrs (n = 5, p≤0.05). Conclusion: VDR is expressed in colonic epithelial cells where its activation by its natural ligand, calcitriol, inhibits agonist-induced Cl- secretory responses. Future studies will aim to determine a possible role for VDR in mediating anti-secretory actions of luminal bile acids, such as LCA. Since Cl- secretion is the primary driving force for fluid secretion in the intestine, our data suggest that VDR may serve as a target for the development of new anti-diarrhoeal agents.