Averaged responses to loud clicks were recorded in the unrectified and rectified masseter EMG of 18 healthy subjects. Unilateral clicks (0.1 ms, 3 Hz, 70-100 dB NHL), delivered during a steady masseter contraction evoked bilateral responses that appeared to consist of two components on the basis of threshold, latency, and their appearance in rectified EMG. The lowest threshold response appeared as a p16 wave (onset 11-13 ms) in the unrectified EMG and corresponded with a 10-12 ms period of inhibition in the rectified EMG. Higher intensity clicks recruited an earlier p11 response in the unrectified EMG (onset 7.0-9.2 ms) that sometimes appeared as initial increase in the rectified EMG prior to suppression. The amplitude of the p11 wave scaled with background EMG level and was asymmetrically modulated by 30° tilt of the whole body. The threshold of the early p11/n15 wave in masseter was the same as the threshold for click-induced vestibulocollic reflexes. Single motor unit recordings demonstrated that responses in masseters corresponded to a silent period in unit firing which began earlier and lasted longer at 100 dB than at 80 dB. We propose that loud clicks induce two partially overlapping short-latency reflexes in masseter muscle EMG: a p11/n15 response which we suggest is of vestibular origin and a p16/n21 response which we suggest is equivalent to the previously described jaw-acoustic reflex.