Enhanced pressor reponses to intravenous doses of tyramine were found in five patients with pheochromocytoma in comparison with 21 subjects with essential hypertension and 11 with normal blood pressure. In two patients with pheochromocytoma who had curative surgery, the tyramine sensitivity returned to normal. The mechanism of the enhanced tyramine sensitivity may be related to increased stores of tyramine-releasable catechol amine at sympathetic nerve endings. The recommended conditions for tyramine testing are discussed, with emphasis on avoidance of its usage in patients receiving monoamine oxidase (MAO) inhibitors. While further evaluation is required, tyramine appears to be a suitable substitute for histamine in the pharmacological diagnosis of pheochromocytoma.