Dog gracilis muscles were removed, enclosed in a plethysmograph and perfused at constant inflow pressure or constant inflow. Circulating blood volumes were measured by the constant infusion technique using RBC-<sup>51</sup>Cr or albumin-<sup>131</sup>l. Control venous pressure averaged 3 mm Hg and elevations (ΔPV) over the range of 5–40 mm Hg were produced. Volume changes were determined during and following ΔPV by plethysmography and by changes in total muscle radioactivity. Changes in total (amount of blood in the tissue), active (circulating), and mobilized vascular volumes were calculated. Active vascular volumes and total vascular volumes increased with venous pressure increments up to 25 mm Hg and then plateaued. Active vascular volumes (indicators) increased by amounts significantly greater than the increases in total vascular volume (plethysmography). Volume changes in the constant flow groups were double those in the constant pressure groups. The mobilized active vascular volume (active vascular volume change minus total vascular volume change) consists of a volume of blood contained in vessels unattainable by the indicators during the control period but which were made available to the indicator by the ΔPV. Mobilized vascular volume averaged 45% of the active vascular volume change. With venous pressure elevation there was an increase in the RBC-<sup>51</sup>Cr volume to albumin-<sup>131</sup>l volume ratio. This suggests a redistribution of red cells with respect to plasma, possibly resulting from reduced plasma skimming.