A blood reservoir is an organ or region containing a significant portion of the blood volume that can be mobilized by the sympathetic nervous system to subserve overall cardiovascular homeostasis. To be effective, the reservoir must be as insensitive as possible to the passive effects of changes in flows or external pressures. Evidence is presented that the splanchnic venous bed fulfills these criteria. Hepatic venous resistance maintains portal pressure and intrinsic hepatic arterial mechanisms maintain total hepatic flow. These mechanisms minimize passive effects of portal flow changes on splanchnic blood volume, whereas enclosure of the liver in the peritoneal cavity minimizes effects of external pressures. Up to 27% of total blood volume can be mobilized from the splanchnic venous bed by sympathetic stimulation. Maintenance of cardiac preload depends on the ability of the central nervous system to control venous compliance and hence to redistribute blood volume between peripheral organs and the cardiopulmonary compartment. Three examples of the role of the splanchnic blood reservoir in overall homeostasis are discussed: hemorrhage, splanchnic nerve stimulation, and responses to epinephrine. Reflex and central control of the splanchnic blood reservoir are discussed but available information is fragmentary.