The ventilatory control system was evaluated in a group of 40 diabetic patients (20 with autonomic neuropathy (AN), and 20 without autonomic neuropathy) and 20 controls. The ventilatory increase in response to transient hypoxia was less in diabetics without AN than in controls, but even weaker in diabetics with AN. This pattern was repeated in the ventilatory response to hypercapnia. There was no correlation between the presence of abnormal respiratory responses and the duration of diabetes.