Studies were performed in anesthetized and awake opossums to define the genesis of resting pressure and opening of the upper esophageal sphincter. Intraluminal pressures were measured with low compliance, perfused catheters. Electromyograms were recorded using miniature concentric bipolar electrodes from the cricopharyngeus, inferior pharyngeal constrictor, middle pharyngeal constrictor, proximal end of the esophagus, and geniohyoid muscles. Upper esophageal sphincter pressure showed radial as well as axial asymmetry; the pressures were higher in awake than in anesthetized animals. Electromyographically, only the cricopharyngeus and inferior pharyngeal constrictor fired continuously at rest and ceased to fire during swallowing. d-Tubocurarine or section of the motor nerve abolished electrical activity in the cricopharyngeus and inferior pharyngeal constrictor and caused marked reduction but not obliteration of upper esophageal sphincter pressure. Contraction of the geniohyoid muscle abolished the remaining pressure in the upper esophageal sphincter. Moreover, contraction of the geniohyoid muscle caused a fall in upper esophageal sphincter pressure, even when the cricopharyngeus and inferior pharyngeal constrictor were kept contracted. These studies indicate: (1) upper esophageal sphincter corresponds mainly to the cricopharyngeus and inferior pharyngeal constrictor; (2) the resting upper esophageal sphincter pressure is attributable to continuous active muscle contraction as well as the elasticity of the surrounding structures; and (3) the opening of the upper esophageal sphincter is attributable to inhibition of the contraction of the cricopharyngeus and inferior pharyngeal constrictor and the forward displacement of the larynx by the geniohyoid muscle.