The endothelium is a monolayer of cells which lines the inside of blood vessels. It responds to several different kinds of agonists by increasing the cytoplasmic concentration of calcium ions ([Ca\sp2+]\sbc). The increase in [Ca\sp2+]\sbc will then trigger the synthesis and/or release of various vasoactive substances. In order to better understand the mechanisms involved in the modulation of cytoplasmic concentration of calcium ions, a mathematical model has been developed. This model includes equivalent circuit descriptions of the plasmalemma and endoplasmic reticulum membranes along with material balance equations for [Ca\sp2+] in different parts of the cytoplasm and intracellular storage organelle. The emphasis of the equivalent circuit membrane models is primarily on describing the calcium permeable channels and the calcium pumps. The model produces good fits to data obtained from several different types of experiments namely those in which intracellular concentration of calcium ions is modulated by: (1) increased phosphatidylinositol turnover due to agonist binding (2) block of the Ca-ATPase pump of the endoplasmic reticulum (3) changes in the extracellular concentration of calcium ions (4) and block of the plasmalemmal Ca influx pathway by SKF96365, in addition to measurements of the cell's membrane potential and rate of efflux of radiolabelled calcium under similar experimental conditions.