The effects of an angiotensin-converting enzyme inhibitor, alacepril, on cardiovascular and plasma catecholamine responses to mental stress were studied. A mental arithmetic test (MAT) was carried out in 9 patients with essential hypertension before and after treatment with alacepril, 25 mg once daily for 2 weeks, and in 9 age- and sex-matched normal subjects. Systolic blood pressure at rest and during MAT and the change in plasma norepinephrine concentrations during MAT were significantly greater in the hypertensive subjects than in the normal subjects. Alacepril significantly reduced systolic blood pressure, both at rest and during MAT, as compared to before treatment in the hypertensive subjects. Furthermore, alacepril significantly attenuated the change in plasma norepinephrine concentrations during MAT (from 127 +/- 76 pg/ml to 66 +/- 42 pg/ml, p < 0.05). These results suggest that alacepril significantly suppresses the augmented cardiovascular and sympathetic nervous responses to mental stress in patients with essential hypertension.