Background: For respiratory resuscitation without devices, the author hypothesized that providing upper airway patency requires lifting the base of the tongue off the posterior pharyngeal wall and that artificial ventilation with intermittent positive pressure using exhaled air, i.e., direct mouth-to-mouth ventilation (MMV), is more effective than back or chest pressure with or without arm lift. MMV leaves the operator's hands free for backward tilt of the head, forward displacement of the mandible, or both. Methods: The author studied 25 sedated, nonintubated adult human volunteers under neuromuscular blockade with succinylcholine for 1-3 h each. One hundred sixty-seven untrained lay persons performed various direct MMV methods after one demonstration. Eighteen trained ambulance rescuers performed back or chest pressure arm-lift methods. Ventilation volumes were recorded during MMV from a calibrated pneumograph and during the manual methods from a taped face mask on a spirometer. Arterial oxygen saturation was monitored by an ear oximeter, and end-tidal carbon dioxide was measured by an infrared analyzer. Results: With the head in the mid position or flexed, airway obstruction occurred in all volunteers, equally in the supine or prone position. With the head tilted backward and the mouth held open, one half to two thirds of the volunteers had an open airway; the remaining volunteers required additional forward displacement of the mandible or a pharyngeal tube. Ninety percent of the lay persons performed MMV effectively. Moderate hyperventilation by the operator achieved normoxemia and normocapnia in the volunteer and moderate hypocapnia in the operator. Apnea-induced moderate hypoxemia was reversed with 5-9 MMVs. In the majority of volunteers, the manual methods caused no ventilation (mostly because of neck flexion), and in others, it caused progressive airway obstruction. In some volunteers, there was valve-like nasopharyngeal obstruction. Conclusions: In coma without a tracheal tube, direct MMV is effective because of the ability of the rescuer to support the head and jaw for upper airway patency and because of controllable high inflation pressures and volumes, whereas manual methods frequently fail to ventilate, mainly because of upper airway obstruction. The author recommends that backward tilt of the head plus exhaled air inflation methods be taught for general use in adults and children.