The effect of low-energy laser (He-Ne) irradiation on the rate of skeletal muscle regeneration after partial excision of the rat gastrocnemius muscle was studied using quantitative histological morphometric methods. The injured zones of the experimental rats were subjected to direct He-Ne laser (632.8 nm wavelength) irradiation (6.0 mW for 2.3 min) immediately following injury and once daily thereafter for 5 days. Muscles that were injured as above and subjected to red or room light irradiation served as a control group. The volume fraction (percent of total volume of injured zone) of the mononucleated cells in the injured zone decreased gradually with time after injury, but more rapidly in the laser irradiated muscles than in the control. At 3 days post-injury the myotubes in the laser-irradiated muscles populated a significantly higher percentage (13.9 +/- 1.1%) of the injured area than in the control muscles (7.8 +/- 1.0%). The volume fraction of the young myofibers in the laser irradiated muscles exceeded 30.6 +/- 2.2% and 49.6 +/- 5.6% at 8 and 11 days post-injury, respectively, while in control muscles these structures comprised only 9.6 +/- 1.0% and 27.2 +/- 3.8% of the injured zone at 8 and 11 days after injury, respectively. It is concluded that He-Ne laser irradiation during the regeneration process promotes muscle maturation in the injured zone following partial excision of the rat gastrocnemius muscle.