Knowledge of the fatigability of the genioglossus muscle is of interest because this muscle prevents pharyngeal collapse, especially during sleep. In the present investigation, signs of fatigue in the genioglossus muscle were studied by measuring the tongue endurance using a force transducer and electromyographic (EMG) activity of the genioglossus muscle in eight nonapnoeic men. Mean absolute EMG values and spectrum analysis were calculated at three levels of submaximal effort. Median frequency and the force:mean absolute EMG value ratio were independent of force level (F = 0.37, P = 0.93; F = 0.35, P = 0.94, respectively) but dependent on effort duration (F = 52, P < 0.0001; F = 16, P < 0.0001). Force:mean absolute EMG value and logarithmic median frequency decreased linearly with respect to time and were similar at the three force levels when time was expressed as a percentage of total test time (F = 0.37, P = 0.93). The decrease in median frequency was ascribable to a larger increase in low- than in high-frequency components, as shown by the significant decrease in the high-frequency:low-frequency ratio (F = 27, P < 0.0001) with time. The method of investigation used in this study allowed detection of the behaviour of the tongue during fatigue and, therefore, should be useful in disorders where mechanical failure of the tongue is suspected, such as the sleep apnoea syndrome or in neuromuscular disorders.