In order to investigate the effects of beta-blockade on haemodynamic response to stress, 2 groups of volunteers received either propranolol or mepindolol sulphate under basal conditions and under the stress of mental arithmetic. In control conditions, increases occurred in heart rate (HR), cardiac output (CO) and systolic and diastolic blood pressure in response to calculation stress. Stress-induced increases in HR and CO were significantly reduced by propranolol. Systolic blood pressure during calculation stress under propranolol was slightly less than in control conditions. However, diastolic blood pressure under stress was higher with propranolol than in the control study. Mepindolol sulphate also reduced HR and CO under calculation stress but it produced no significant change in diastolic pressure. A significant increase in plasma adrenaline occurred under stress. The level was not altered by propranolol but was reduced to less than 50% of the untreated stress value by mepindolol. Since beta-blocking agents suppress an increase in heart rate in stressful situations but either fail to prevent or even potentiate increased diastolic pressure, their use in patients without hypertension should be reserved for those showing pathological stress reactions. They do not appear to be indicated for the management of stressful situations in general.