Cardiovascular and plasma catecholamine responses to metoclopramide (MCP), a dopamine antagonist, were examined in 5 patients with pheochromocytoma, 12 patients with essential hypertension (EHT) and 9 normotensive (NT) subjects who displayed symptoms suggestive of pheochromocytoma on a constant daily intake of 100mEq sodium and 80mEq potassium. Significant pressor responses to intravenous doses of 5mg of MCP, which produced no serious pressor episodes and no other undesirable side effects, were found only in the patients with pheochromocytoma, in contrast to the subjects with EHT and NT who tended to display slight depressor responses. After curative surgery for pheochromocytoma, the MCP-induced pressor effects returned to normal. Furthermore, the enhanced pressor effects of MCP in the patients with pheochromocytoma were associated with increased plasma norepinephrine (NE) concentrations. However, the plasma epinephrine (E) concentrations remained unchanged after the MCP injection. Thus, this dose of MCP appears to be a more suitable vasopressor provocative agent in the pharmacological diagnosis of pheochromocytoma compared to currently used agents.