1. Studies with a sensitive radioenzymatic assay for plasma noradrenaline suggest there is a selective overactivity of the sympathetic nervous system in essential hypertension.
2. Serotonin turnover in the mesenteric vessels is approximately twice that of noradrenaline and it is suggested that serotonin may interact with noradrenaline to maintain vascular resistance.
3. Methodology which allows the study of local sympathetic turnover in nuclei of the central nervous system and in peripheral blood vessels is described. This approach has been used to study non-innervated sympathetic turnover observed in phaeochromocytoma.