Aims: To clarify the effects of experimental trapezius muscle pain on the spread of pain and on jaw motor function. Methods: In 12 male subjects aged 25 to 35 years, experimental pain was induced in the superior border of the trapezius muscle by injecting 0.5 mL of hypertonic (6%) saline. The control infusion consisted of a 0.5-mL isotonic (0.9%) saline solution. Pain intensity was evaluated on a visual analog scale (VAS). An experimental (EX) and a control (CT) injection were administered to the subjects in a randomized sequence. Results: Pain intensity as scored on the VAS increased immediately after the EX injection and decreased gradually after reaching a peak of 68.0 ± 16.1 mm at 60 seconds after injection. The VAS scores in the EX condition were significantly higher than after the CT condition from 30 to 330 seconds after injection (P , .05, analysis of variance [ANOVA]). Mean (± SD) maximal unassisted mouth opening before injection in the EX condition was 54 ± 5.7 mm and decreased immediately after the injection, reaching a low of 47.8 ± 5.1 mm. A gradual recovery to normal was then observed. This reduction of mouth opening in the EX condition was significant compared with the CT condition from immediately after the injection to 60 seconds after the injection (P , .05, ANOVA). According to the subjects, pain spread most often to the infra-auricular zone (n = 6), and the posterolateral part of the neck (n = 10). Conclusion: The present results suggest that experimental trapezius muscle pain can spread over a wide area and is also accompanied by a temporary reduction of mouth opening. Keywords: experimental pain, jaw motor function, limited mouth opening, pain spreading, trapezius muscle