The ventilatory function of rectus and oblique abdominis muscles was studied by means of surface electromyography in 14 patients with severe obstructive lung disease and 9 healthy control subjects. While the control subjects did not use the abdominal muscles in sitting position at rest, one third of the patients showed an active expiration and half of them a continuous activity already during quiet spontaneous breathing. During voluntary hyperventilation mobilized the majority of the patients the abdominal muscles at the mean ventilatory rate of 22.9 l/min in comparison with control subjects who did not activate their muscles until the ventilation increased beyond 50 l/min. Both muscles contracted in the course of slow expiration of vital capacity, significantly earlier in the patients than in the controls. Also, the antagonist inspiratory activity on abdominal muscles during maximal inspiration of VC and hyperventilation was more pronounced in the patients. It is concluded that in the patients with severe emphysema the abdominal muscles are mobilized more readily during different ventilatory manoeuvres and even at rest than in the healthy subjects.