The pulmonary vasoconstrictor responses to U-46619 and PGF2a are calcium dependent. The purpose of this investigation was to determine to what extent extracellular and intracellular calcium pools are utilized during the dose-dependent pulmonary vasopressor responses induced by multiple doses of U-46619 and PGF2a. Increasing doses of these agonists were administered to isolated rat lungs perfused with Krebs-Ringer bicarbonate (KRB) or KRB not containing CaCL2. The data indicate that U-46619 uses predominantly extracellular calcium at low doses (0.1 microgram) and depends solely on intracellular calcium at the highest dose (0.4 microgram). In contrast PGF2a appears to use depletable intracellular calcium stores to achieve contraction.