The discharge of muscle spindle afferents from monkey spindle afferents from monkey jaw-closing muscles was studied during mastication of natural foods by extracellular recording from the fibers or cell bodies of the tract and mesencephalic nucleus of the fifth nerve. In all, 39 muscle afferents were studied. The spindle associated with 18 of the afferents was positively identified by the afferent's response to gentle, localized palpation of either the temporalis or masseter muscle. Discharge patterns were observed during mastication, and in the majority of cases the qualitative passive response characteristics of the spindle afferent were determined. During steady chewing spindle afferent discharge typically paused briefly during the initial rapid upward part of the chewing cycle. Firing generally began as the jaw slowed its upward movement, and firing rates during the slow grinding portion of the upward movement were within the range of 50-80 spikes/s. All spindles exhibited a brisk discharge during the opening movement, typically within the range of 100-150 spikes/s. One-third of the spindle afferents exhibited a brief, high-frequency burst of firing at the very beginning of the opening movement, presumably as a result of stretch applied to a spindle just previously subjects to fusimotor excitation. Although the results of the study make it clear that spindles in jaw-closing muscles are coactived along with the extrafusal muscle fibers, the fusimotor bias does not seem capable of sustaining discharge in the face of rapid shortening of the muscle. Furthermore, the fact that discharge rate during opening, when the jaw-closing motoneurons are quiescent, is much higher than at any part of the closing cycle, when the motoneurons are active, suggests that the muscle spindles cannot provide the primary excitatory drive to the motoneurons.