1. A radiotracer method, based on the measurement of noradrenaline spillover to plasma, has been developed to estimate simultaneously total, and organ-specific, sympathetic nervous system tone in humans.

2. Total, and renal, noradrenaline spillover to plasma was measured in four patients with essential hypertension. Renal noradrenaline spillover was 0.42 ± 0.25 nmol/min (mean ± sd), constituting approximately 16% of total noradrenaline spillover (2.58 ± 0.20 nmol/min).

3. There is a need to study regional sympathetic nervous tone, of organs particularly involved in blood pressure regulation, in patients with essential hypertension. Local increases in sympathetic tone might not be detected by biochemical tests of overall sympathetic nervous system activity.

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