In order to test the effect of stress on injection-site blood flow and blood glucose control, 16 C-peptide negative patients were studied on a stress day, when a 30-min Stroop Colour-Word Test was completed, and a control day, when a cartoon was shown. Unmodified insulin was injected subcutaneously into the thigh before the test, and injection-site blood flow measured throughout the experiment with a thermal clearance probe. Blood glucose and plasma free insulin, glucagon, growth hormone, cortisol, and catecholamines were measured at intervals before, during, and after stress. Patients showed a significant overall rise in injection-site blood flow with the Stroop test from 4.1 (SD 1.6) to 5.2 (1.8) ml 100-g-1 min-1 (increase 38.1 (37.8) %, p less than 0.001). There was no overall significant difference between stress and control days in blood glucose or plasma free insulin levels, with differences in mean blood glucose levels during stress between the 2 days varying from -4.2 mmol l-1 to +6.3 mmol l-1 in individual patients. The increase in injection-site blood flow with stress correlated significantly with the increase in plasma free insulin concentration both during (r = 0.55) and after stress (r = 0.71). Differences in blood glucose concentration between stress and control day for each patient showed strong correlation with differences in plasma free insulin both during (r = -0.73) and after stress (r = -0.79). Differences in counter-regulatory hormones occurred but correlated poorly with blood glucose difference. Thus, stress affects the blood flow at the injection site, and this in turn affects insulin absorption. Most of the blood glucose response to stress is explained by changes in free insulin concentration.