Vascular endothelial growth factor (VEGF) chronically increases microvascular permeability, compliance and vessel diameter. To determine the signalling pathways by which VEGF exerts these effects, we investigated the role of Ca2+ influx and mitogen-activated protein kinase (MAPK) phosphorylation on the increase in hydraulic conductivity (Lp), diameter and compliance in mesenteric microvessels in the anaesthetised frog (Rana species). The VEGF-mediated chronically increased permeability was attenuated by co-perfusion of VEGF with 5 mM NiCl2, previously shown to inhibit Ca2+ influx. MAPK phosphorylation inhibition by PD98059 did not affect the chronic increase in Lp. To determine whether other agonists which increased Ca2+ influx also chronically increased Lp, the effect of ATP perfusion on chronic Lp was measured. ATP perfusion also chronically increased Lp. The chronic increase in Lp was therefore dependent on an initial transient Ca2+ influx, and not MAPK activation, and was not unique to VEGF stimulation. Inhibition of Ca2+ influx did not inhibit the increase in microvascular diameter or compliance brought about by VEGF. Both these increases were inhibited by PD98059. The VEGF-mediated increase in compliance and diameter was therefore dependent on MAPK activation, not on Ca2+ influx. The chronic increase in Lp stimulated by VEGF perfusion 24 h previously was reduced when the vessel was perfused with 5 mM NiCl2. The sustained, high Lp was therefore dependent on Ca2+ influx. The endothelial cell calcium concentration ([Ca2+]i) of vessels previously perfused with VEGF or ATP, and with a chronically increased Lp, was not significantly increased compared to [Ca2+]i of endothelial cells in vessels before agonist perfusion These experiments show that VEGF acts through different pathways to stimulate increased permeability and compliance. The data are consistent with the hypothesis that VEGF chronically increases Lp through an acute stimulation of Ca2+ influx, but increases compliance and diameter by acute stimulation of the MAPK signalling pathway. They also suggest that the increase in Lp is dependent on a sustained Ca2+ influx, even though the endothelial [Ca2+]i is not raised.