The electrical activity of the arytenoideus muscle, a vocal cord adductor, was measured in 14 normal adult humans during progressive isocapnic hypoxia and progressive hyperoxic hypercapnia. Electromyograms of the arytenoideus were obtained with intramuscular hooked-wire electrodes implanted by means of a fiber-optic nasopharyngoscope. Correct placement of the electrodes was confirmed by discharge patterns during voluntary maneuvers. In three of the subjects, respiratory-related arytenoideus activity was not present during quiet breathing or chemical stimulation. During quiet breathing in the 11 other subjects, the arytenoideus exhibited phasic activity during expiration and usually tonic activity throughout the respiratory cycle. Phasic and tonic arytenoideus activity decreased under hypoxic and hypercapnic conditions. At higher levels of chemical stimulation in many subjects, short abrupt bursts of activity were frequently present at the transitions between inspiration and expiration. To determine the mechanical effect of the latter electromyographic findings, arytenoideus activity and fiber-optic images of the glottic aperture were simultaneously recorded in nine additional normal adult human subjects during progressive hyperoxic hypercapnia. The short abrupt bursts of arytenoideus activity were usually associated with a decrease in glottic aperture, although no change and an increase in glottic aperture were observed in individual subjects. The results suggest that the arytenoideus muscle may have an important role in the control of ventilation in normal human subjects.