The nociceptive trigeminal inhibitory (NTI) splint has been claimed to decrease the electromyographic (EMG) activity of jaw-closing muscles and relieve symptoms of various types of temporomandibular disorders (TMD) and bruxism. The present study was designed to address the question about EMG-changes during sleep. Ten patients (age: 23-39 years) with a self-report of tooth-grinding during sleep were recruited. Patients were examined at baseline and after each treatment period with the use of the Research Diagnostic Criteria for TMD. A portable EMG-device was used to record EMG-activity from the masseter muscle during sleep. The patients received two 2-week splint treatments in a randomized cross-over fashion; an NTI splint and a standard flat occlusal splint (OS). EMG data were analysed according to published criteria. Using a 10% of maximum clenching EMG-activity cut-off threshold to determine the number of EMG-events h-1 of sleep, the NTI splint was associated with a significant reduction (9.2 ± 3.2 events h-1) compared with baseline EMG (19.3 ± 4.0; anova: P = 0.004, Tukey post hoc: P = 0.006), whereas there were no differences between the OS (16.2 ± 4.7) and baseline EMG (19.2 ± 4.1; P = 0.716). There were no effects of either NTI or OS on clinical outcome measures (anovas: P > 0.194). This short-term study indicated a strong inhibitory effect on EMG-activity in jaw closing muscles during sleep of the NTI, but not the OS. However, the EMG-activity was not directly related to clinical outcome. Further studies will be needed to determine long-term effects and possible side effects of the NTI splint. © 2007 Blackwell Publishing Ltd.