Tight junctions (TJ) between retinal pigmented epithelial (RPE) and retinal endothelial cells maintain the outer and inner blood-retinal barrier, and the breakdown of these barriers is associated with retinal diseases. Vascular endothelial growth factor (VEGF) increases vascular permeability and is thought to be involved in age-related maculopathy. However, to date, little is known about the effect of VEGF on RPE cell junctions. We have investigated the effect of VEGF on TJ formation by examining two essential proteins, ZO-1α⁺ and ZO-1α⁻. Cultured vascular endothelial cells in the presence of 5 ng/ml VEGF significantly down-regulate ZO-1α⁺ and ZO-1α⁻ transcripts and proteins with significant loss of their trans-epithelial resistance (TER). Immunoconfocal analysis with an anti-ZO-1 antibody has confirmed the relocation of ZO-1 protein from membrane to cytoplasm. By contrast, in the presence of 5 ng/ml VEGF, cultured RPE cells (ARPE19 and RPE51) significantly up-regulate ZO-1α⁺ and ZO-1α⁻ transcripts and proteins resulting in a significant increase in their TER. Subsequent immunoconfocal analysis has demonstrated increased ZO-1 membrane assembly in VEGF-treated RPE cells. Thus, VEGF has a dual capability with respect to the regulation of the expression of some TJ proteins at the transcriptional and post-translational levels depending on cell type.