The effect of muscular contraction on vessel diameter was studied in the arteriolar network of the exteriorized cat sartorius muscle during normal and elevated vascular tone. Dilation during 4 Hz motor-nerve stimulation was proportionately greatest in the third-order (transverse) arterioles and in vessels immediately upstream and downstream (P less than 0.01). This pattern of dilation was maintained with increased contraction frequency (30 Hz) and during concurrent sympathetic nerve stimulation (8 Hz). The pattern of constriction with sympathetic nerve stimulation alone showed a similar trend with the greatest response in the third-order and adjacent vessels. A model developed to estimate the resistance distribution in the arteriolar network, using data from earlier micropressure and vascular architecture studies in the sartorius muscle, allowed calculation of the resistance change during muscle contraction and sympathetic stimulation. Model predictions indicate that the third-order and adjacent vessels are the greatest site of resistance with both normal and elevated vascular tone. Thus these vessels were the site of greatest resistance change during muscle contraction. The more proximal, arcade vessels made lesser contributions to overall resistance changes, whereas the most distal, fifth-, and sixth-order arterioles appear not to be important in this regard. These findings indicate the third-order, transverse, arterioles are of special importance in regulating blood flow in the sartorius muscle.