Dog gracilis muscles were removed, enclosed in a plethysmograph and perfused at an inflow pressure of 110 mm Hg. Venous outflow pressure was 3 mm Hg. Circulating blood volumes were measured by the constant infusion technique using RBC-<sup>51</sup>Cr (9 muscles) or albumin-<sup>131</sup>I (11 muscles). Volumes were calculated from the infusion and from the washout of the indicator. Skeletal muscle contraction was produced by stimulation of the gracilis nerve stump with supramaximal stimuli at 2-15 Hz for 15-60 sec. Volume changes during and following the stimulations were measured by plethysmography and by changes in total muscle radioactivity. Inflow and outflow were also measured. Changes in interstitial fluid volume were calculated as the difference between total tissue and vascular volume changes. Blood volume increased at all contraction rates. Total tissue volume initially decreased during the contraction period and then returned to control. Extravascular volume decreased markedly during the initial period of contraction and then increased above control, returning to the control level following the end of the stimulation. The tissue pressure markedly increased during the contractions causing fluid movement into the vascular system. Minimal loss of labeled albumin during the period of contraction indicated capillary permeability was not altered.