Pharyngeal obstruction in patients with obstructive sleep apnea (OSA) is thought to result from decreased upper airway muscle tone during sleep. The goal of the present study was to estimate the role of the tongue muscles in maintaining pharyngeal patency during sleep. Using non-invasive, sub-lingual surface electrical stimulation (ES), we measured tongue protrusion force during wakefulness and upper airway resistance during sleep in seven healthy subjects and six patients with OSA. During wakefulness, ES produced similar protrusion forces in healthy subjects and patients with OSA. ES of the anterior sublingual surface, causing preferential contraction of the genioglossus, resulted in smaller effects than combined ES of the anterior and lateral surface, which also stimulated tongue retractors. During sleep, trans-pharyngeal resistance decreased and peak inspiratory flow rate increased from 319+/-24 to 459+/-27 and from 58+/-16 to 270+/-35 ml/sec for healthy subjects and OSA patients, respectively (P<0.001). However, ES was usually unsuccessful in reopening the upper airway in the presence of complete apneas. We conclude that non-invasive ES of the tongue improves flow dynamics during sleep. Combined activation of tongue protrusors and retractors may have a beneficial mechanical effect. The magnitude of responses observed suggests that in addition to the stimulated muscles, other muscles and/or forces have a substantial impact on pharyngeal patency.