1. Free normetadrenaline concentrations in plasma of patients with phaeochromocytoma were elevated 20-1000-fold as compared with values in primary hypertensive subjects. 2. Spinal fluid normetadrenaline was 1.8-16% of the simultaneous plasma values in patients with phaeochromocytoma, whereas levels in spinal fluid exceeded those of plasma in primary hypertension. 3. Spinal fluid noradrenaline was 7.9-58% of simultaneous values of plasma noradrenaline in patients with phaeochromocytoma, whereas values in plasma and cerebrospinal fluid were similar in primary hypertension. 4. Plasma noradrenaline and normetadrenaline levels were both elevated in veins from regions subsequently found to harbour chromaffin tumours. 5. The excessive amounts of normetadrenaline produced in phaeochromocytoma enters plasma but not spinal fluid and can be used for diagnosis and localization of phaeochromocytoma.