The effects of subablative Q-switched Nd-YAG laser irradiation on the nerve fiber population of the sciatic nerve in rat were estimated using a quantitative method. The acute and cumulative effects were evaluated separately. When 5-minute laser irradiation was applied to the sciatic nerve, there was a differential decrease in the number of active fibers in slow and fast fiber classes. Fast fibers were insensitive to the radiation as the number of fibers was not altered significantly except at high radiating energies. The amount of decrease in each fiber class was not strictly proportional to the radiating laser energy. The cumulative effects show an exponential decay in fiber number after radiation sequence of gradually incremented laser pulse energy. Similarly, the impairment of fast fiber classes was less significant than slow fiber classes. At high cumulative optical energy, the number of fibers in fast fiber classes started decreasing whereas fiber population in slow fiber classes increased. We postulate that the impairment of the node of Ranvier was the dominant mechanism at low energy radiation, whereas the change in the cable properties of internodes resulted in the shift of fast fibers to slower fiber classes.