Vascular endothelial growth factors (VEGFs) are known to increase vascular permeability. VEGF-A acts on two receptor tyrosine kinases, VEGF receptor-1 (VEGF-R1 or flt-1) and VEGF receptor-2 (VEGF-R2, flk-1 or KDR). VEGF-C acts only on VEGF-R2 on vascular endothelial cells, whereas placental growth factor-1 (PlGF-1) acts only on VEGF-R1. The effects of perfusion of these receptor-specific proteins on hydraulic conductivity (L<sub>p</sub>) was measured in frog mesenteric capillaries. The effect of PlGF on L<sub>p</sub> was not conclusive, and overall fluid flux did not increase during that time. VEGF-C acutely and transiently increased L<sub>p</sub> (4.5 ± 0.9-fold), which was more obvious in a subset of vessels, in a similar manner to that reported for VEGF-A. In the subset of vessels in which VEGF-C significantly increased L<sub>p</sub> acutely, there was a sustained 12-fold increase in L<sub>p</sub> 20 min after perfusion, but this was not seen in those vessels which did not respond acutely to VEGF-C, or in vessels exposed to PlGF-1. L<sub>p</sub> was also increased 24 h after perfusion with VEGF-C, but not with PlGF-1. Western blot analysis showed that VEGF-R1 and VEGF-R2 are both present in frog tissue. These data show that the VEGFs that stimulate VEGF-R2 chronically increase L<sub>p</sub>, but not those that stimulate VEGF-R1 only. This supports the hypothesis that chronic increases in microvascular permeability induced by VEGF are mediated via activation of VEGF-R2 rather than VEGF-R1.