We studied masseter muscle function in 15 hemiplegic patients. Direct motor responses to stimulation of the masseteric nerve were normal. Voluntary activity at maximum strength was reduced on the paretic side. The jaw jerk was symmetric, but facilitation by teeth clenching showed on the nonparetic side only. Masseter silent periods were ill-defined and prolonged in recordings from the paretic side. The second silent period, but not the first, was reduced in size when the paretic side was stimulated, and showed an increased habituation. Though the masticatory nucleus receives bilateral innervation, the contralateral hemisphere exerts a predominant control on voluntary activity and multisynaptic reflexes.