Vascular permeability is regulated by endothelial intracellular Ca²⁺ concentration [Ca²⁺]_i . To determine whether vascular permeability is dependent on extracellular Ca²⁺ influx or release of Ca²⁺ from stores, L_p was measured in single perfused frog mesenteric microvessels in the presence and absence of Ca²⁺ influx and store depletion. Prevention of Ca²⁺ re-uptake into stores by sarcoendoplasmic reticulum Ca²⁺ ATPase (SERCA) inhibition increased L_p in the absence of extracellular Ca²⁺ influx. L_p was further increased when Ca²⁺ influx was restored. Depletion of the Ca²⁺ stores with ionomycin and SERCA inhibition increased L_p in the presence and the absence of extracellular Ca²⁺ influx. However, store depletion in itself did not significantly increase L_p in the absence of active Ca²⁺ release from stores into the cytoplasm. There was a significant positive correlation between baseline permeability and the magnitude of the responses to both Ca²⁺ store release and Ca²⁺ influx indicating that the Ca²⁺ regulating properties of the endothelial cells may regulate the baseline L_p. To investigate the role of Ca²⁺ stores in regulation of L_p, the relationship between SERCA inhibition and store release was studied. The magnitude of the L_p increase during SERCA inhibition is significantly and inversely correlated with that during store release by Ca²⁺ ionophore implying that the degree of store depletion regulates the size of the increase on L_p. These data show that microvascular permeability in vivo can be increased by agents that release Ca²⁺ from stores inthe absence of Ca²⁺ influx. They also show that capacitative Ca²⁺ entry results in increased L_p and that the size of the permeability increase can be regulated by the degree of Ca²⁺ release.