Plasma catecholamine levels have been determined before, during and after operation in seven consecutive patients operated upon for pheochromocytoma. The method used permits the simultaneous determination of norepinephrine, epinephrine and dopamine. All patients had increased levels of one or several catecholamines while hypertensive. Two of seven patients had normal levels of catecholamines while they were normotensive. Spontaneous or provoked attacks strongly increased plasma catecholamine levels. The present method may prove to be of value as a simple method to ascertain or exclude pheochromocytoma. During operation plasma norepinephrine and epinephrine levels increased up to 600fold when the tumor was manipulated. The unpredictable rapid and large variations in levels of catecholamines found during operation suggest preoperative alpha-adrenergic receptor blockade and monitoring of central circulation.