This study examined the temporal effects of bolus volume on closure of the laryngeal vestibule at the arytenoid to epiglottic base and the mobile portion of the epiglottis, the temporal relationships between these levels of airway closure and cricopharyngeal opening for various bolus volumes, and the mechanisms responsible for these two levels of airway protection during deglutition. Closure of the laryngeal vestibule progressed inferiorly to superiorly at all bolus volumes. Duration of closure of the airway at the arytenoid to epiglottic base increased systematically with bolus volume, as did the duration of descent of the epiglottis below horizontal. Closure at the arytenoid to epiglottic base occurred earlier in relation to maximal laryngeal elevation as bolus volume increased. In contrast, descent of the epiglottis to horizontal and the temporal relationship between closure of the airway at the arytenoid to epiglottic base and cricopharyngeal opening were independent of bolus volume. These findings indicate a tightly organized neural program for some pharyngeal swallow events with systematic variability with volume in other pharyngeal events, possibly modulated by afferent input from the periphery. The neuromuscular mechanisms responsible for closure of the airway at the arytenoid to epiglottic base and at the mobile epiglottis appear to be quite different. Closure at the arytenoid to epiglottic base is apparently under direct neural control by active anterior tilting of the arytenoid cartilage and posterior projection of the epiglottic base as the larynx elevates, whereas epiglottic downward movement to closure is the biomechanical effect of hyolaryngeal movement, downward bolus movement, and tongue base retraction.(ABSTRACT TRUNCATED AT 250 WORDS)