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Following a case of mannitol-induced respiratory and circulatory collapse, the effects of hyperosmolar injections on pulmonary arterial pressure, systemic blood pressure, and cardiac output were studied in dogs. The injection of 20 ml of 10% NaCI into the pulmonary artery increased pulmonary arterial pressure and decreased systemic blood pressure by approximately 50% of control values. Injections of solutions of equal hyperosmolar strength, 50 ml of 25% mannitol or 50 ml of 4% NaCI into the pulmonary artery produced no significant elevation of pulmonary arterial pressure, but were associated with comparable decreases in systemic blood pressure. When allowed to vary, cardiac output increased with injections of all three hyperosmolar solutions, yet was still accompanied by falls in systemic blood pressure as large as when cardiac output was held constant. Vagotomy did not prevent these changes in systemic and pulmonary arterial pressure nor the increase in cardiac output. After five to 10 injections, the decreases in systemic blood pressure with any of the solutions and the increases in pulmonary arterial pressure with 10% NaCI disappeared and further injections were without effect. It is concluded that administration of mannitol probably does not cause pulmonary edema due to fluid overload, nor does it cause heart failure as evidenced by increases in pulmonary arterial pressure. However, rapid injection may cause a fall in blood pressure and may on occasion be accompanied by bronchospasm especially in sensitive subjects.