Injury to a mammalian peripheral nerve is accompanied by a restorative process that is manifested after a delay. This process is expressed morphologically by the emergence of new nerve fibers. Restoration of function occurs when the regenerating fibers reconnect with the target organ. Because of the low rate of fiber elongation, the denervated target is partially degenerated by the time that the regenerating fibers approach it. To prevent such an atrophy, one must find a way to prevent the degeneration of the nerve, to speed up regeneration, or to maintain the target during the period of nerve degeneration. In the present work, we examined the potential of treatment with low energy laser radiation for improving regeneration or preventing degeneration of mammalian peripheral nerve after injury. After repeated injury for 20 consecutive days, treatment of the sciatic nerve of the rat with low energy laser (He-Ne, 17 mW) caused a significant increase in the amplitude of the action potential recorded in the corresponding gastrocnemius relative to the action potential of injured but not treated nerves. The action potential of the injured sciatic nerves that were laser-irradiated increased to values close to that of a noninjured nerve. The studies include follow-up for 1 year after the injury. This electrophysiological manifestation of the effect of laser treatment on injured nerves was accompanied by a diminution of the size of the scar tissue from these nerves. Yet to be resolved is whether these two phenomena (i.e., electrophysiological and morphological responses) coincide or whether they relate to each other.(ABSTRACT TRUNCATED AT 250 WORDS)