Background/purpose: Increased occlusal jaw opening may lead to some changes in the bite force and electromyographic (EMG) activities of the jaw muscles. This in situ study was conducted to determine the immediate influence of three different occlusal vertical dimensions on the maximum bite force and EMG activities of the masseter and anterior temporalis muscles. Materials and methods: Five healthy subjects participated in the study. Two maxillary occlusal splints of 2 and 4 mm thickness were fabricated, and their use created, respectively, 8 and 10 mm of intermolar distances in the first molar region with a strain-gauge-mounted metal arch. EMG activities of the muscles and the maximum bite force were simultaneously recorded using an EMG device and two miniature strain-gauge transducers. Recordings were made in the first molar region without a splint, but with two splints to produce 6, 8, and 10 mm of intermolar distances. Results: EMG activities of both muscles decreased with increased jaw opening; however, the differences were not statistically significant. The highest maximum bite force was found at a 6-mm intermolar distance, which significantly differed from those at 8- and 10-mm intermolar distances (P < 0.05). The most efficient bite force was exerted with 6 mm of intermolar distance. The least EMG activity of both muscles was found with 10 mm of intermolar distance. Conclusion: An increase in the vertical thickness of the splint to 10 mm may provide an immediate effect of reducing masseter and anterior temporalis muscle hyperactivity. © 2012, Association for Dental Sciences of the Republic of China. Published by Elsevier Taiwan LLC. All rights reserved.