Electromyographic activity was recorded in the masseter muscle to investigate whether the firing characteristics of single motor units (SMUs) were affected by muscle pain. Capsaicin was injected into the masseter to induce pain. The interspike interval (ISI) and recruitment threshold of SMUs were measured while subjects performed isometric contractions at 5, 7.5, 10, 15, and 20% of maximum voluntary contraction. All subjects were able to maintain a stable isometric force during pain, but the mean ISI was significantly increased without changing the recruitment threshold. In all the experimental conditions, the firing frequency increased with increasing force, and SMUs recruited at low force fired at higher rates for all force levels. These results suggest that acute stimulation of nociceptive muscle afferents inhibits SMU activity without changing recruitment order in the homonymous muscle. © 2000 John Wiley & Sons, Inc. Muscle Nerve 23: 1219–1226, 2000.