Several studies have demonstrated that nocturnal bruxism and related symptoms can be relieved through nocturnal electromyogram (EMG) feedback. The effects, however, are not always long lasting and the mechanism of suppression is not understood. It was the object of this study to examine more closely the manner in which nocturnal feedback works in suppressing bruxism. Chart recordings were made of nocturnal masseter EMG activity in five bruxism subjects. Baseline recordings were made in the subject's home followed by 10 or more nights of feedback treatment. Treatment involved sounding a 300 mW tone when EMG activity exceeded about 20 micro V for more than 1 second. All subjects showed a decrease in the duration of bruxism. The decrease in bruxism was due to a reduction in the duration of bruxism episodes rather than a change in the number of episodes, i.e. rather than reducing the probability of an event starting, nocturnal feedback appears to simply suppress the activity once it is initiated. This provides little evidence of learning.