We have studied the effects of Q-switched Nd:YAG laser irradiation on transmission of neural impulses in sensory nerve fibers in anesthetized rats and cats. Laser light was applied to dorsal roots (rat, cat) and to the sciatic nerve (rat) at increasing pulse energies ranging from 10 to 100 mJ/pulse for 5 minutes each. Compound action potentials recorded from dorsal roots and the sciatic nerve in response to high intensity electrical stimulation during laser application at increasing pulse energies showed a progressive preferential reduction of the slow late component of the electrically evoked response. Preliminary data from multifilament recordings from dorsal roots in cats demonstrated that conduction in small slow conducting fibers was blocked at lower laser pulse energies than in fibers with faster conduction velocities. These results imply, that laser light might have differential effects on slow versus fast conducting sensory nerve fibers. It is most likely that the preferential effect of laser irradiation on slow conducting fibers is mediated by photothermal mechanisms, since temperature increased substantially during laser application.