400 sleep-apnoeic patients with an intact autonomic nervous system underwent twenty-four-hour electrocardiography with simultaneous polygraphy recording at night. At onset of sleep apnoea all showed progressive bradycardia, followed by abrupt tachycardia on resumption of breathing. The electrocardiographic pattern, which is identifiable by computer analysis, can be used as a screening tool for sleep apnoea; it was not seen in controls without sleep apnoea syndrome. A subgroup of patients with sleep apnoea and impairment of autonomic nervous control of the heart (heart transplants, autonomic neuropathy, Shy-Drager syndrome) did not show the cyclical heart rate pattern. In obstructive sleep apnoeic patients with normal autonomic nervous function, atropine sulphate blocked the pattern by eliminating the bradycardia component, while 100% oxygen, even at high rates of administration, caused only moderate blunting of the heart rate variation. The electrocardiographic changes observed in sleep apnoea syndrome are therefore mediated by the autonomic nervous system; hypoxia is not the only factor involved.