The electromyographic activity of the glossal, suprahyoid, infrahyoid, and pharyngeal muscles was examined during spontaneous respiration in rabbits anesthetized with ketamine hydrochloride. This activity was then correlated with phases of the respiratory cycle. Our findings indicate that the overwhelming majority of the muscles comprising these groups show activity that increased during inspiration and returns to the background level during expiration and the end-expiratory pause. The exceptions are the inferior pharyngeal constrictor muscle, which demonstrates increased activity during expiration and the end-expiratory pause, and the stylohyoid major and digastric muscles, whose activity was not modulated with respiration. In general, the results obtained under ketamine anesthesia are in agreement with the studies on a more limited number of muscles in humans during sleep or in animal studies utilizing light anesthesia. Furthermore, the use of ketamine avoids the central suppressant effects produced by barbituate anesthesia. It has been argued that the upper airway muscles are rhythmically active during respiration to maintain the patency of the upper airway. Both the number of muscles that are rhythmically active and their strict correlation with specific phases of the respiratory cycle suggest that the forces exerted on the upper airway are complex and that peak tension is generated during inspiration. Further studies are required to evaluate the effects of ketamine anesthesia on these upper airway muscles before this rabbit model can be utilized to examine respiratory disorders of the upper airway.