We have recently demonstrated that the number of small sensory neurons of the A-δ- and C-fiber group in lumbar dorsal root ganglia labeled with horseradish peroxidase (HRP) is selectively decreased 7 days after Nd:YAG laser irradiation of the tibial nerve in the rat. In contrast, the number of large diameter sensory neurons was not affected by laser application. In an attempt to clarify the fate of motoneurons after laser irradiation of their peripheral axons, the numbers of lumbar motoneurons retrogradely labeled with HRP 7 days after Nd:YAG laser irradiation of the tibial nerve have been determined in rats. Our results show that the number of HRP-labeled motoneurons in lumbar segments L6 to L3 is not altered to a significant extent after laser irradiation of their peripheral axons (laser-treated side, 767 ± 10 cells vs control side, 808 ± 19; n = 5, mean ± SEM). In addition, no difference was detected in the mean value or the distribution of soma cross-sectional areas of labeled motoneurons on the laser-treated side and the control side. Specifically, the numbers of HRP-labeled small diameter motoneurons, which are presumably γ in type and have a conduction velocity similar to sensory neurons of the A-δ group, were not affected by laser application. Possible mechanisms of the differential vulnerability of sensory neurons as compared to motoneurons of similar size are discussed. © 1993 Academic Press, Inc.