Low power laser irradiation has been reported to cause biological effects due to the photochemical and/or photophysical action of the radiation. This study determined quantitatively if transcutaneous low power laser irradiation can affect the regeneration of the rat facial nerve. The facial nerve was crushed unilaterally in anesthetized rats and transcutaneously irradiated daily with a laser beam directed at the area of the crush injury. Laser treatment began on the day of the crush injury and was continued daily for 7, 8, or 9 days. Preliminary experiments determined the most effective wavelength, laser power, length of irradiation, and treatment schedule. The wavelengths examined were 361, 457, 514, 633, 720, and 1064. The laser powers and lengths of irradiation examined ranged from 8.5 to 40 mW and 13 to 120 min. Irradiation treatment was done daily, on alternating days and on the first 4 days postcrush. The most effective laser parameters for the low power treatment included daily irradiation with a helium-neon (HeNe) or argon pumped tunable dye laser a wavelength of 633 nm, with a power of 8.5 mW for 90 minutes (45.9 J, 162.4 J/cm2). The number of horseradish peroxidase (HRP) labeled neurons in the facial motor nucleus was used as an assay of the degree of regeneration. In rats in which the facial nerve was crushed but not irradiated, the average number of HRP labeled neurons in the facial nucleus was 22 on day 7 postcrush, 54 on day 8, 116 on day 9, and 1,149 on day 10. After HeNe or argon pumped tunable dye laser irradiation, the average number of HRP-labeled neurons increased to 34 on day 7 postcrush, 148 on day 8, and 1,725 on day 9. There was a statistically significant difference between the control and irradiated rats on day 9 postcrush (P < 0.01). These data indicate that transcutaneous low power irradiation with the lasers and parameters involved in this study increased the rate of regeneration of rat facial nerve following crush injury.