Insulin hypoglycaemia was induced three times in 6 insulin-dependent, hypertensive diabetics: before instituting antihypertensive long-term treatment and after obtaining a satisfactory blood pressure with either alprenolol or metoprolol given in a randomized order. The blood glucose concentration (1.6-1.9 mmol/l) at which the hypoglycaemia necessitated intravenous administration of glucose was almost identical on all three occasions. During hypoglycaemia the systolic and diastolic blood pressures increased significantly by a mean maximal rise of 27/14 mmHg on alprenolol treatment, but remained unchanged on metoprolol. The responses of adrenaline, noradrenaline, cortisol, and growth hormone did not differ significantly on the three occasions. None of the beta-adrenergic drugs counteracted the early hormone defence mechanisms in hypoglycaemia and the signs of hypoglycaemia were not masked. The haemodynamic response was altered only by the non-selective (alprenolol) and not by the selective beta-adrenergic blocking agent (metoprolol).