Intravenous infusions of highly concentrated NaCl (2,400 mosmol/l; infused volume 4 ml/kg; equivalent to 10% of shed blood), given to lightly anesthetized dogs in severe hemorrhagic shock, rapidly restore blood pressure and acid base equilibrium toward normality. No appreciable plasma volume expansion occurs for at least 12 h, indicating that fluid shift into the vascular bed plays no essential role in this response. Initial effects wee sustained indefinitely; long term survival was 100%, compared to 0% for a similar group of controls treated with saline. Hemodynamic analysis of the effects of hyperosmotic NaCl showed that these infusions substantially increase mean and pulse arterial pressure, cardiac output and mesenteric flow, whereas heart rate was slightly diminished. These effects immediately follow infusions with no tendency to dissipate with time (6-h observation). We conclude that hyperosmotic NaCl infusions increase the dynamic efficiency of the circulatory system, enabling it to adequately handle oxygen supply and metabolite clearance, despite a critical reduction of blood volume.