Wister rats at the age of 1, 3, 6 days were irradiated with GaAlAs diode lasers (805 nm) at several power levels ranging from 13 to 200 mW for 15 min at the point located above the cerebellar culmen. They were sacrificed 6 hours after irradiation, and their cerebella were removed and processed for microscopic observation. In the mid-sagittal sections through the vermis, incidences of pyknotic cells and mitotic cells among matrix cells in the external granular layer along the primordial culmen were estimated. Cerebellar temperaturesunder irradiated spots in rats at the age of 3 days were also measured by a needle-shaped thermocouple. Cerebellar temperatures in rats irradiated at power levels more than 100 mW were significantly higher than those of sham-irradiated group. No significant difference from sham-irradiated groups was observed in the incidences of pyknotic and mitotic cells in all rats irradiated with diode lasers. In rats at the age of 3 days irradiated with diode laser of a clinical instrument (NIR monitor) of Shimadzu Corp., no significant change in temperatures and the incidences of pyknotic and mitotic cells was found in their cerebella. These facts suggest that diode laser irradiation at power less than 200 mW does no cause cell death in the neonatal rat brain.
