The purpose of the present study was to test the hypothesis that increasing muscle contraction frequency, which alters the duty cycle and metabolic rate, would increase the contribution of the contractile phase to mean venous blood flow in isoltaed skeletal muscle during ryhthmic contractions. Canine gastrocnemius muscle (n=50) was isolated and 3-min stimulation periods of isometric, tetanic contractions (200ms train duration with 50 Hz stimuli) were elicited sequentially at rates of 0.25, 0.33, and 0.5 contractions/sec. The oxygen uptake, tension-time integral (index of work), and mean venous blood flow increased significantly (p<0.05) with each contraction frequency (blood flow being 71±6, 84±9, and 107±11 ml^.100 g^-1.min^-1, respectively). Venous blood flow during both the contractile (106±6, 139±8, and 145±8 ml^.100 g^-1.min-1) and non-contractile phases (64±3, 78±4, and 91±5 ml^.100 g^-1.min^-1) increased with contraction frequency. Although developed force and the duration of the contractile phase was never significantly different for a single contraction during the 3 contraction frequencies, the amount of blood expelled from the muscle during an individual contraction increased significantly with contraction frequency (0.24±0.03, 0.32±0.02, 0.36±0.03 ml^.N^-1.min^-1, respectively). This increased blood expulsion per contraction, coupled with the decreased time in the non-contractile phase as contraction frequency increased, resulted in the contractile phase contribution to mean blood flow becoming significantly greater (21±4, 30±4, and 38±6%) as contraction frequency increased. These results demonstrate that the percent contribution of the muscle contractile phase to mean venous blood flow becomes significantly greater as contraction frequency increases, and that this is in part due to increased blood expulsion per contraction.