The effect of acute mental stress on insulin sensitivity was evaluated in ten IDDM patients, studied on two occasions (test day and control day) in random order and separated by a period of 1-3 weeks. Mental stress was evoked by a modified filmed version of Stroop's CWT for 20 min. On the control day, the patients were resting quietly during the corresponding period. Insulin sensitivity was estimated by an insulin (0.4 mU.kg-1 x min-1)-glucose (4.5 mg.kg-1 x min-1)-infusion test (IGIT) for 6.5 h. Mental stress evoked significant responses for adrenaline, cortisol and GH, their respective peak values being 0.27 +/- 0.05 nmol/l, 426 +/- 27 nmol/l and 7.6 +/- 1.8 micrograms/l, as well as increases in systolic and diastolic blood pressure and pulse rate The steady-state blood glucose levels, i.e. the mean blood glucose levels 3-6.5 h after the start of the IGIT, were significantly higher after stress, compared with those on the control day, 10.6 +/- 1.5 vs 8.7 +/- 1.4 mmol/l, p = 0.01, demonstrating impairment of the insulin sensitivity by mental stress. It is concluded that acute mental stress induces a state of insulin resistance in IDDM patients, which can be demonstrated by an IGIT to appear 1 h after maximal stress and to last more than 5 h.