In the present paper we report our experience on the utility of basal plasma catecholamine (CA) measurement and of the clonidine-suppression test in the diagnosis of pheochromocytoma. Basal noradrenaline (NA) and adrenaline (A) were assayed in plasma samples of 27 subjects affected by pheochromocytoma. When compared to basal values obtained in hypertensive patients without pheochromocytoma, one or both the CA resulted pathologically elevated in all patients except one. The response to the clonidine-suppression test was evaluated in 41 hypertensive patients suspected of having a pheochromocytoma measuring plasma NA and A in basal conditions and 2 and 3 h after oral administration of 300 micrograms clonidine. Extensive laboratory and instrumental findings confirmed the presence of pheochromocytoma only in 12 patients. Among the other 29 patients basal plasma CA resulted higher than normal in 4 patients. In patients without pheochromocytoma clonidine induced a significant fall in both NA and A plasma levels. The decrease in NA was observed in each patient. The 12 patients with pheochromocytoma showed a pathological elevation of one or both the CA. In this group clonidine did not significantly suppress plasma CA. The individual responses were extremely variable. Our data confirm the validity of plasma CA measurement as a diagnostic tool for the diagnosis of pheochromocytoma. The results of the clonidine-suppression test were generally confirmatory of the basal CA plasma values but in the 4 hypertensive patients without pheochromocytoma who showed basal plasma CA higher than normal clonidine resulted a useful tool in excluding the presence of pheochromocytoma.