Tyramine hydrochloride has been proposed for use as a provocative test for pheochromocytoma. It has been evaluated in 46 hypertensive patients and 58 normotensive controls by members of the US Public Health Service Cooperative Study. Positive tests were obtained preoperatively in two patients with elevated catecholamine and vanilmandelic acid (VMA) levels with proved pheochromocytomas. Seven patients with labile blood pressure responses also had a positive tyramine test. In the latter group, results of catecholamine and VMA determinations were all normal. The tyramine test is a useful and safe addition to the screening procedures in the evaluation of hypertensive patients for remediable causes of elevated blood pressure. Twenty-four-hour urine collections for catecholamine or VMA determinations still are required to confirm the diagnosis of pheochromocytoma.