Several studies suggest a biomodulatory influence of low-level laser irradiation in the inflammatory and reparative processes of dental tissues. The aim of this study was to investigate the stimulatory effect of 808-nm laser irradiation on root development of rat molars and also to evaluate the histological reaction of pulp and periapical tissues. Twenty-four 30-day-old Wistar male rats were randomly assigned to three-time and five-time laser therapy groups. After initial x-ray, using mammography equipment, laser energy was applied at a wavelength of 808 nm (2 J/cm(2), 100 mW, 20 s) to the midroot area of the lower molars of one side of mouth at repeated intervals of the 48 h. The animals were killed 1 day after the final treatment, and root length development of the experimental samples was compared to contra-lateral non-irradiated molars using mammography. The histological reaction of the pulp and periapical tissue was evaluated under light microscopy. Root development was more advanced in irradiated groups than in the non-irradiated controls (p < 0.001). No significant differences, however, could be found between the root development changes in the three-time and five-time laser therapy groups (p > 0.05). Histological findings showed that the occurrence of secondary cement formation was significantly higher in the irradiation groups compared to the controls (p = 0.003). However, there were no statistically significant differences for the frequencies of pulp hyperemia, periodontal ligament fiber organization, or lamina dura remodeling between the groups (p > 0.05). Under the conditions used in this study, 808-nm low-level laser accelerates the rat molar root development in the presence of favorable histological reactions.