To simulate increased sympathoadrenal activity adrenaline was infused in normotensive subjects to achieve plasma adrenaline concentrations similar to those seen after myocardial infarction or hypoglycaemia. Adrenaline was infused after pretreatment for five days with labetalol 200 mg twice daily or placebo given in a random order. The rise in systolic blood pressure and the fall in diastolic blood pressure observed after the infusion of adrenaline (0.06 micrograms/kg/min) were prevented by labetalol and no increase in blood pressure was seen. Adrenaline infusion after pretreatment with placebo caused a profound fall in the serum potassium concentration (4.12-3.20 mmol(mEq)/l). Pretreatment with labetalol completely blocked adrenaline induced hypokalaemia (3.92-3.95 mmol(mEq)/l). Adrenaline induced T wave flattening and QTc prolongation were also prevented by labetalol. Thus labetalol can prevent the electrocardiographic, haemodynamic, and hypokalaemic effects of increased circulating adrenaline in man. The combination of alpha and beta blockade appears to be required to block the haemodynamic effects of adrenaline, and labetalol may, therefore, be useful in controlling both the metabolic and circulatory responses during increased sympathoadrenal activity.