1. Arterial plasma catecholamines and haemodynamic status were simultaneously studied in 27 patients with fixed essential hypertension (WHO I-II). 2. Total arterial plasma catecholamines were found to be directly related to mean arterial pressure and to calculated total peripheral resistance, and they were inversely related to stroke index. 3. The degree of total peripheral resistance reduction after alpha-receptor blockade (phentolamine 10 mg intravenously) and the degree of mean arterial pressure reduction after combined alpha- and beta- receptor blockade (labetalol 100 mg intravenously) were directly related to pretreatment plasma catecholamine concentrations. 4. These findings support the view that the sympathetic nervous system, as evaluated by plasma catecholamine concentration, may have an important role in maintaining hypertension in a subgroup of patients, contributing mainly to the degree of peripheral vasoconstriction.