The objective of this study was to evaluate a new quantitative electrophysiologic method and instrumentation for assessing the regenerative process in peripheral nerve injuries. The method was based on spectrum analysis of myoelectric signals. Myoelectric data were obtained longitudinally from affected muscles of the upper limb in patients with traumatic brachial plexus injuries, and quantitatively subjected to on-line analysis. Bipolar surface recordings were made during voluntarily mediated motor unit activity at both a low level of activity (nonfatiguing state) and a maximal level (fatiguing state). The results over time for the low-level activity have shown a significant increase in mean frequency of the myoelectric signal in some muscles, a significant decrease in other muscles, and no change in still other muscles. A significant increase in amplitude was observed in every instance. During the maximal level of activity there was a reduction in mean frequency and the amount of reduction was shown to increase over time. The results are useful in indicating the occurrence of peripheral sprouting, axonal regrowth, of central reorganization, and in indicating improved metabolic activity in a muscle. It is concluded that the method is reliable in the study of the regenerative process.