1. Normetanephrine was measured in small samples of plasma and urine of hypertensive patients and normal volunteers (age 20–60 years) by a specific radioenzymatic assay with bovine adrenal phenylethanolamine N-methyltransferase and tritiated S-adenosylmethionine.
2. Noradrenaline was measured simultaneously in plasma and urine.
3. Plasma normetanephrine and noradrenaline concentrations varied in direct proportion to activation or suppression of sympathetic nerve function.
4. Both plasma and urinary normetanephrine concentrations were elevated in patients with phaeochromocytoma.
5. Plasma normetanephrine concentrations were related to plasma noradrenaline concentrations of hypertensive subjects.
6. Plasma normetanephrine and noradrenaline concentrations and urinary normetanephrine excretion rates were increased in some young patients with primary hypertension, suggesting that sympathetic nerve hyperactivity is a pathogenic factor in these patients.