In this study, pain was induced in the masseter muscle by tonic infusion of hypertonic saline (5%) for up to 800 seconds in 12 healthy men. Subjects continuously scored the pain intensity on a 10-cm visual analogue scale. Mastication ipsilateral and contralateral to the infusion side was quantitatively assessed with the use of jaw-tracking and electromyograph recordings of jaw-closing muscles before, during, and after periods of constant muscle pain intensity. The maximum voluntary occlusal force (MVOF) during short static contractions also was monitor ed. Jaw movements and electromyographic data were divided into single masticatory cycles and analyzed on a cycle-by-cycle basis to account for intercycle variability. In all subjects, tonic infusion (mean VAS ± SE, 4.6 ± .3 cm). MVOF was significantly affected by muscle pain (P .0005), with significantly lower MVOF during pain compared to prepain and postpain (P .05). In a significant number of masticatory cycles, the averaged electromyograph activity of all jaw-closing muscles during their agonist function was decreased for both ipsilateral and contralateral painful mastication (P .05). These electromyographic changes are probably a reflection of the natural bilateral recruitment pattern of jaw-closing muscles during mastication. Significant changes in jaw movements during painful mastication could not be detected with the present jaw-tracking device, but further studies with more accurate and sensitive devices are needed.