The following recordings were made during spontaneous rhythmic jaw movements in the anesthetized guinea pig: jaw movement in the vertical plane, electromyograms from the digastric muscle (a jaw opener), and intracellular recordings from motoneurons and spindle afferent fibers of the jaw-closer muscles. The rapid jaw-opening phase of these rhythmic movements is characterized by a marked increase in activity in the spindle afferent fibers of the jaw-closer muscles and a coincident pronounced hyperpolarization of the membranes of the jaw-closer motoneurons. It is proposed that the excitatory spindle afferent input evoked by passive stretch of the jaw-closer muscles resulting from a centrally programmed contraction of the jaw-opener muscles impinges upon closer motoneurons that have been hyperpolarized as part of te central program. The hyperpolarization inhibits the motoneurons and nullifies the effect of the spindle afferent input thereby assuring relaxation of the closer muscles and an unimpeded rapid jaw-opening movement.