Peripheral circulatory effects of insulin were studied in the diabetic patients with and without autonomic neuropathy. Forearm blood flow, calf venous volume and calf venous distensibility were measured by strain gauge plethysmography. In the diabetic patients with autonomic neuropathy, mean blood pressure fell from 96 +/- 5 to 88 +/- 5 mmHg after an intravenous injection of 4 U of monocomponent insulin (p less than 0.001). Forearm vascular resistance decreased from 53.99 +/- 8.29 to 45.88 +/- 7.76 mmHg X ml-1 X 100ml-1 X min-1 after insulin (p less than 0.01). Insulin increased calf venous volume from 1.20 +/- 0.19 to 2.23 +/- 0.44 ml/100ml (p less than 0.05) and calf venous distensibility from 0.039 +/- 0.004 to 0.082 +/- 0.016 ml/mmHg (p less than 0.05). In contrast, in the diabetic patients without autonomic neuropathy, there were no significant changes in the mean blood pressure, forearm vascular resistance, calf venous volume and calf venous distensibility. Symptoms of hypoglycaemia did not occur in any patient. These results suggest that insulin has a vasodilator action on both resistance and capacitance vessels, which may be one of the main factors in insulin-induced hypotension.