Vasopressin (AVP) has trophic effects in the rat distal colon, increasing pericryptal myofibroblast growth and reducing the colonic crypt wall permeability. We have examined whether these effects of AVP can be reproduced in vitro, using the myofibroblast CCD-18Co cell line to study proliferation and the colonic epithelial cell line T84 to study the expression of tight junction proteins. The possible role of the platelet derived growth factor A (PDGFA) as mediator of myofibroblasts effects on the expression of junctional proteins was also studied. Cell proliferation was quantified from 5-Bromo-2′-deoxyuridine incorporation, the expression of tight-junction proteins β-catenin and claudin IV by Western blot and PDGFA expression by real time PCR. Results are expressed as means ± S.E.M. (n=6), compared by ANOVA. AVP (10 nM, 24 h) stimulated CCD-18Co proliferation by 60% (p<0.05) and increased PDGFA expression by 20%, and both effects were prevented when Manning peptide, a V1 receptor antagonist, and Tolvaptan, a V2 receptor antagonist, were present in the incubation medium. Similar effects were observed when exogenous PDGFA was added. In addition, when myofibroblasts were treated with vasopressin and pre-incubated with anti-PDGF or anti-PDGF receptor antibodies, the AVP effects were prevented. AVP had no direct effects on the expression of junctional proteins by the T84 cells; however, both β-catenin and claudin IV were increased when the cells were incubated for 24 h with conditioned medium (CM) from myofibroblasts stimulated by AVP. The CM increased T84 proliferation by 46% (p<0.05) and these effects were prevented by anti-PDGFA antibodies (p<0.05). The CM from CCD-18Co cells treated with AVP increased the expression of β-catenin (p<0.05) and claudin IV by 25% (p<0.05). These results indicate that changes in colonic permeability during dehydration are mediated by PDGFA secreted by myofibroblasts in response to raised AVP.