1. The response of plasma noradrenaline, arterial blood pressure and heart rate to sustained handgrip at 30% of maximal voluntary contraction was studied in patients with long-term juvenile-onset-type diabetes mellitus and healthy subjects of comparable age. 2. There was no significant difference between the intensity and duration of handgrip in diabetic patients and healthy subjects. 3. Sustained handgrip produced an increase in plasma concentration of noradrenaline both in diabetic and healthy subjects but the response in the diabetic subjects was significantly less. 4. The increase in systolic blood pressure during handgrip was significantly greater in diabetic subjects than in normal subjects. The increases in diastolic and mean blood pressure did not differ significantly. 5. The increase in heart rate during handgrip was greater in healthy subjects than in diabetic subjects. The response was smaller in diabetic patients with retinopathy than in the patients without retinopathy. 6. The sustained handgrip test may be useful for the diagnosis of abnormal sympathetic nervous system and haemodynamic responsiveness in diabetic patients.