Sodium notroprusside (Nipruss), in comparison with halothane and trimetaphan a very suitable hypotensive drug, was examined for blood pressure lowering efficency and possible side effects. For this purpose 7 patients (group 1) underwent "classical" hypotension during neurosurgical procedures. In 5 cases (group 2) nitroprusside infusion was needed for therapy of hypertensive crisis during vascular surgery. In all patients the hypotensive action was prompt and sufficient. Low blood pressure was accompanied by a 22.5% rise of heart rate and a 66% fall of central venous pressure. The development of mild metabolic acidosis seemed to be without clinical importance. The doses employed ranged from 3 to 13 (grpup 1) and from 1.5 to 3.1 gamma/kg/min (group 2). Sodium nitroprusside proved a very useful hypotensive drug, fitting well with the needs of modern anaesthesia.