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To evaluate autonomic nervous function by power-spectral analysis of heart-rate variability in salt-sensitive and non-salt-sensitive patients with essential hypertension under the conditions of low and high salt intakes.The blood pressures, heart rates, and electrocardiogram R–R intervals of 20 hypertensive patients were measured at intervals of 30 min during a 24 h period using a portable recorder (TM-2425) on the last day of the high- (250 mmol NaCl/day) and low-salt (25 mmol NaCl/day) diet periods. The patients whose 24 h average mean blood pressures were increased by more than 10% by the high salt intake were defined as salt-sensitive (n = 10); the other patients were considered non-salt-sensitive (n = 10). Power-spectral analysis of R–R intervals was performed to obtain the low-frequency component (0.05–0.15 Hz) and the high-frequency component (0.15–0.40 Hz).The average 24 h blood pressure in the salt-sensitive patients was increased by the high salt intake [by 19.1 ± 2.0/9.1 ± 0.8 mmHg (mean ± SEM)], whereas the heart rate did not change. In contrast, the increase in 24 h blood pressure in the non-salt-sensitive patients caused by the high salt intake was not significant and the heart rate was decreased significantly by the high salt intake (by 5.9 ± 1.4 beats/min). The high-salt diet increased significantly the high-frequency component and decreased the low-frequency: high-frequency component ratio both during the daytime and during the night-time for the non-salt-sensitive patients. In contrast, the high-frequency component and the night-time low-frequency: high-frequency component ratio of the salt-sensitive patients did not respond to dietary salt manoeuvres.Responses of the parasympathetic and sympathetic nervous systems to dietary salt manoeuvres were blunted in salt-sensitive patients. These altered modulations of the autonomic nervous system may contribute to the salt sensitivity of the blood pressure in patients with essential hypertension.