Previous investigators have reported that transient forced expiration is accompanied by abduction of the vocal cords. To further investigate the laryngeal response during voluntary forced vital capacity maneuvers, intramuscular electromyographic recordings were obtained in 25 normal adult humans from three intrinsic laryngeal muscles: the posterior cricoarytenoid (PCA), a vocal cord abductor, and the thyroarytenoid (TA) and arytenoideus (AR), both vocal cord adductors. All three muscles exhibited sustained activation throughout most of forced expiration from total lung capacity. Forced inspiration from residual volume was associated with a further increase in PCA activity and a marked decrease in adductor muscle activity. To determine the net effect of these electromyographic changes on vocal cord position, simultaneous fiberoptic recordings of vocal cord movement were obtained in five of the subjects. The angle formed by the vocal cords at the anterior commissure was used to assess glottic aperture size. Glottic angle progressively decreased from peak expiratory flow to the end of forced expiration. The angle was 56 +/- 13 degrees (SD) at peak expiratory flow, 34 +/- 4 degrees after forced expiration of 90% of the vital capacity, and 7 +/- 7 degrees at end-expiration. The results indicate that forced expiration is associated with marked activation of not only the PCA but also laryngeal adductor muscles. During forced expiration, the glottis does not decrease below its size during quiet breathing until exhalation of about 75% of forced vital capacity.