1. By using the rebreathing method, ventilatory responses to inhaled CO2 were studied in patients with diabetic neuropathy. 2 Their general respiratory functions including vital capacity (VC) and forced expiratory volume in 1 s (FEV 1.0) were within normal ranges and ventilatory responses to CO2 in the range 5.32--5.99 kPa were not significantly different between patients and normal subjects. However, ventilation in the patients was significantly less than that in normal subjects when the alveolar partial pressure of CO2 (PACO2) was above 6.65 kPa. 3. The slope of the CO2 response curves obtained during hyperoxic rebreathing was significantly less in the patients than in the normal subjects. In the patients CO2 response curves with 15% (v/v) O2 were either the same or had lower slopes than those measured with 100% O2. 4. The results suggest that both peripheral and central chemosensitivity, including the activity of the respiratory centre, are reduced in patients with diabetic neuropathy.