Five patients with systemic arterial hypertension occurring in the postoperative period and resistant to sodium nitroprusside are described. The hypertension was associated with evidence of increased activity of the autonornic and renin-angiotensin systems. Sympathetic blockade using the ganglion blocking agent, pentolinium, enabled the arterial pressure to be controlled adequately in all patients. This was associated with a decrease in catecholamine concentrations, plasma renin activity and angiotensin II concentrations.