1. No evidence was found of a ;postural reflex' in ducks. Neither the position of the head nor the water temperature affected the cardiac response to diving.2. In ducks with access to air through a tracheal cannula, submersion did not invariably cause apnoea until the water level reached the glottis. Heart rate was closely related to respiratory frequency, and bradycardia did not occur during submersion unless there was a reduction in respiratory frequency or a cessation of ventilation altogether.3. When apnoea and bradycardia did occur during submersion, the first inspiration upon surfacing was 2-3 times larger than normal and was accompanied by an instantaneous rise in heart rate.4. Atropinization or cold block of the vagus abolished diving bradycardia. Only one vagal trunk was involved in cardiac chronotropic control at any one time. This vagal trunk also appeared to be more important in control of respiratory frequency.5. beta-adrenergic receptor blockade did not affect either diving bradycardia or post-dive tachycardia.6. The results show that the cardiac chronotropic response both during and after submergence is controlled solely by changes in parasympathetic vagal activity.