When the face of diving animals comes into contact with water, the diving response is elicited, resulting in bradycardia, peripheral vasoconstriction and ventilatory depression. The cardiovascular aspects of the diving response have been described in man; little is known about the ventilatory depression in man. Compresses with water of various temperatures were applied to the cheeks of 16 normal volunteers and of two hyperventilating subjects. Ventilation was measured with a spirometer or with a pneumatic thoracic transducer: an accordion shaped balloon, strapped around the thorax. When the ventilation was measured with the spirometer via mouth-piece and nose-clip, the diving response caused ventilatory depressions to 60-70% of control. Water temperatures of 10, 20 and 37 degrees C had no significantly different effects from one another on ventilation. When the face of the subjects was left free of nose-clip and mouth-piece and the ventilation was measured by a thoracic transducer, the diving response caused a ventilatory depression to approximately 60% of control. The ventilatory depression did not adapt within 5 min. The two subjects who hyperventilated reacted qualitatively and quantitatively the same as the other subjects.