Atypical tongue muscle activity during sleep may contribute to the development of obstructive sleep apnea (OSA). Inspiratory genioglossus (GG) muscle activity was investigated in 10 OSA adults and 4 symptom-free controls. On the basis of overnight monitoring during nonREM sleep, the duration of the inspiratory GG activity and the total GG activity cycle is shorter in patients with OSA. The duration of inspiration and the duration of one total respiratory cycle is also shorter in patients with OSA. The commencement time lag between inspiratory GG activity and the onset of inspiration is shorter in patients with OSA during nonapneic breathing which indicates that inspiratory GG activity is activated relatively later in these patients. Furthermore, the inspiratory GG activity occurs after inspiration during an apnea, but the timing of GG activity onset progressively advances during the apnea. Earlier GG reactivation occurs before inspiration during the first nonoccluded breath at the end of an apnea. During subsequent tidal breathing, the timing of the GG onset progressively decreases after the onset of inspiration until the next obstructive apnea occurs. This observation suggests that the timing relationship between GG inspiratory activity and inspiratory effort is of physiologic importance in the pathogenesis of OSA. Furthermore, it may explain why dental appliances, such as the tongue retaining device, are highly effective in the resolution of OSA in selected patients.