Measurement of plasma methoxyamines is aimed to provide a reliable plasma marker readily available for any patient suspected of having a pheochromocytoma. The present HPLC method is able to detect methoxyamine amounts as low as 0.2 nmoles/l and allows the determination of both free or conjugated methoxyamines (metanephrine MN and normetanephrine NMN) in normal subjects, treated or untreated patients and a fortiori in pheochromocytoma. The analytical specificity is excellent. Among most usual antihypertensive drugs, only conversion enzyme inhibitors and diuretics induce in some patients a moderate increase of NMN with minor consequence in values interpretation. The kidney plays a major role in the blood clearance of methoxyamines and any alteration of renal function is associated to the increase of plasma methoxyamine levels. Plasma methoxyamines have a long half-life and are long-lasting integrated markers of catecholamine secretion; at variance with plasma free catecholamines, they are able to afford the presence of a secreting pheochromocytoma whatever the clinical presentation--asymptomatic or paroxysmal form between the crisis. The diagnostic sensitivity (at least 98%) is equal to that of urinary methoxyamines, far higher than plasma or urine catecholamines or urine VMA (60 to 70%).