To elucidate the mechanism of blood pressure control in essential hypertension (EH), plasma norepinephrine concentrations (PNE), plasma renin activity (PRA), cardiac index (CI) and total peripheral resistance (TPR) were determined in normal subjects and those suffering from EH while resting and after standing with furosemide. The results were as follows: 1) PNE were 204 +/- 100 pg/ml (mean +/- S.D.) in normal subjects and 313 +/- 257 pg/ml in EH. PNE in EH were not significantly greater than in normal subjects, but in some patients with EH, PNE were above normal. 2) In both normal subjects and those suffering from EH, PNE and PRA showed a positive correlation (r = 0.45, p less than 0.05; r = 0.52, p less than 0.001; respectively). 3) In low renin EH, the response of PNE to the stimulation of standing with furosemide was significantly greater than in normal renin EH (p less than 0.05). 4) PNE in EH were independent of both TPR and CI. 5) There was a positive relation between PRA and CI (r = 0.44, p less than 0.05), and a slight negative relation between PRA and TPR (r = 0.36, 0.05 less than p less than 0.1). These results suggest that sympathetic nervous hypertonicity may be responsible for the activation of the renin-angiotensin system and that the interaction of both the sympathetic nervous system and the renin-angiotensin system may play an important role in the elevation of blood pressure. In low renin EH, the response of PNE to the stimulation of standing with furosemide may be normal and blood pressure may be controlled by both the sympathetic nervous system and the renin-angiotensin system.