Postoperative arterial hypertension was treated with nitroprusside or esmolol in a prospective randomized study. If treatment failed, patients were first switched over to the other study drug, and then to a combination of both. Renin, angiotensin II, epinephrine, and norepinephrine were measured before, 1, and 17 h after operation. esmolol was effective in all cases of treatment (n=6) whereas nitroprusside was ineffective in two out of seven patients (not significant), who were finally treated effectively with the combination of the study drugs. Whenever monotherapy failed, both drugs together were effective in reducing the postoperative activation of the sympathetic and renin-angiotensin systems.