The masseter and the mylohyoid nerves were examined using electron microscopy after unilateral destruction of the trigeminal motor nucleus by kainic acid injection in order to evaluate the distribution of sensory and motor fibers in masticatory muscle nerves. The surviving axons were counted and their diameters measured, and the results were compared with data for control nerves. The diameters of the control masseter nerves fibers were bimodally distributed with peaks at 2-5 and 7-9 micrometers. The nerve fiber diameters on the injected side, i.e., afferent fibers, also showed a bimodal distribution with peaks at 2-4 and 6-9 micrometers. On the other hand, the fiber diameter spectrum of the motor fibers, which was estimated from the loss of these fibers after kainic lesions, was unimodal within ranges of 1-16 micrometers. Consequently, the largest fibers of the masseter nerve were estimated to be motor fibers. A comparison of the distribution of diameters and conduction velocities of the afferent fibers indicates that both primary and secondary spindle afferents are mostly included in the larger diameter group with the peak at 6-9 micrometers. The diameters of fibers of the common mylohyoid nerve trunk showed a bimodal distribution, similarly to those of the masseter nerve. However, the analysis of the peripheral branches of the mylohyoid nerve revealed that the largest afferents in this nerve were of cutaneous origin, since muscle afferents of the anterior digastric and mylohyoid muscles have small diameters. These findings suggest that muscle spindles are absent or few in these two suprahyoid muscles in the cat.