Physical and biological effects of two CO2 laser machines were compared. The two laser machines were an older one built in-house (TL #2) and a new machine (BST-18E) commercially purchased. Thermal-sensitive paper and thermosensors were used to measure the temperature profiles of the laser beam. Compared to the TL laser, the BST laser had lower maximal power, less beam divergence, and more evenly distributed thermal energy within the beam circle. The biological effects compared were the tail flick ratio, cortical evoked potential, and evoked response of ensemble cortical neurons. The TL laser (45 mJ) had an ED50 four times less than that of the BST laser (180 mJ). When comparable laser energy levels were used, similar patterns of evoked cortical field potentials and ensemble neuronal responses composed of short- and long-latency components were seen in the primary somatosensory cortex of the rat.