To determine the effects of vasomotor tone on intrathoracic and splanchnic blood volume, the distribution of radioactively (99mTc) labeled erythrocytes was recorded by whole body scintigraphy before and during peridural anesthesia (PDA) in eight supine men. The radioactivity was recorded with a gamma camera and its distribution determined in the thorax, abdomen, and limbs. Arterial and central venous pressure, heart rate, and calf volume and flow also were measured. During PDA with a sensory block up to T4/5, radioactivity increased only in the denervated legs (+9.9 ± 2.3% SE), whereas it decreased in all other regions, i.e., in the thorax (-8.1 ± 1.2%), the innervated upper limbs (-10.6 ± 4.0%), and in the splanchnic vasculature (-5 ± 1.7%). However, in two of the subjects, after an initial decrease, splanchnic blood content increased while intrathoracic blood volume decreased further. The effects of PDA on thoracic and splanchnic filling could be duplicated by the sequestration of about 500–600 ml of blood in both legs. In supine humans, high peridural anesthesia evokes the same decrease in intrathoracic blood volume as orthostasis. This seems to be counteracted by a reflex decrease in filling of the denervated muscle and skin areas and also by a constriction of the splanchnic vasculature by an unknown mechanism. Potential circulatory collapse may ensue when the vasoconstrictor response fails in the splanchnic circulation.