The percent of vital capacity at which phonation is normally initiated was investigated. Thirty normal young male and thirty normal young female adults breathed oxygen in a closed circuit respirometer in a standing position. Vital capacity, expiratory reserve, tidal volume for breathing, and phonation volume were determined for each subject. The mean percent of vital capacity at which phonation is normally initiated was found to be 49.7 for males and 50.3 for females. These values approximate end-inspiratory volumes during quiet breathing in the upright posture. breath stream dynamics; communication; respiratory muscle activity; electromyography; respiration Submitted on July 13, 1964