This study was undertaken to investigate the pathogenesis of brachial plexus stretching injuries at an intensity level not severe enough to cause avulsion injury. While we performed traction on 64 forelegs of 32 rats, we evaluated changes in the blood flow in the extrinsic and intrinsic microvascular systems of the brachial plexus. While we laterally stretched the brachial plexus during 80 degree shoulder abduction, we measured the blood flow at the bifurcation of the brachial plexus and at the median nerve with the hydrogen washout technique. During weak traction the blood flow decreased markedly in the extrinsic system, causing an imbalance in the two systems. In the median nerve, however, no such imbalance occurred. On histologic examination the axon and myelin in the brachial plexus and the median nerve showed no morphologic change. However, in parts of the brachial plexus we noted hypertrophic connective tissue or granulomatous inflammation in tissue surrounding the extrinsic system. The extrinsic system's apparent susceptibility to injury by acute traction may be a factor in the pathogenesis of the brachial plexus stretching injuries.