The effect of number, frequency, and timing of HeNe (6.0 mW; 31.2J/cm2) and Ga-As-diode (average power at 2.82 Hz-0.005 mW) laser irradiations on the process of muscle regeneration at 14 days following cold injury to the toad gastrocnemius muscle was investigated using histomorphometric methods. The volume fraction (percent of total injured area) of mononucleated cells, myotubes and degenerated fibers was 10 +/- 1%, 0%, and 4 +/- 1%, respectively, in the HeNe laser irradiated muscles (5 irradiations every alternate day, beginning on the 4th day after injury), whereas in the control nonirradiated muscles, these values were significantly higher comprising 57 +/- 2% (P < 0.01), 11 +/- 1% and 10 +/- 2% (P < 0.05), respectively. The volume fraction of young myofibers in injured areas that were subjected to the same laser irradiation regime was 8.6-fold significantly higher (P < 0.01) than their volume fraction in control muscles. The histomorphometric results were the same for injured zones of muscles that were laser irradiated only once, on the 9th day postinjury, and for those that received five consecutive irradiations every alternate day. Muscle regeneration was equally promoted by single Ga-As-diode laser or HeNe irradiation. Multiple irradiations of Ga-As-diode laser caused some pathological changes in the newly formed muscular structures. It is concluded that the process of skeletal muscle regeneration is markedly promoted by low energy laser irradiations, but that the effect depends on the number, timing, and frequency of irradiations and the type of laser used.