ObjectiveThe purpose of this study was to investigate the effects of salt loading on circadian patterns of blood pressure (BP) and sympathetic nervous activity.Subjects and methodsSeventy-six patients with essential hypertension were hospitalized and placed on a low-salt diet (2 g/day) for 7 days followed by a high-salt diet (20–23 g/day) for another 7 days. On the last day of each salt diet, 24 h ambulatory BP, plasma noradrenaline concentrations, urinary noradrenaline excretion, plasma renin activity (PRA) and plasma aldosterone concentration (PAC) were measured. Patients whose average mean BP was increased by more than 10% by salt loading were assigned to the salt-sensitive (SS) group (n= 44); the remaining patients, whose mean BP was increased by less than 10%, were assigned to the non-salt-sensitive (NSS) group (n= 32).ResultsSalt loading converted the circadian pattern of BP from dippers, whose mean BP during the night-time was decreased by more than 10% from the daytime BP, to non-dippers in the SS group but not in the NSS group. A nocturnal decrease in plasma noradrenaline concentration was unaffected after salt loading in the NSS group but dampened in the SS group. The night-time/daytime ratio of urinary noradrenaline excretion, which was increased after salt loading in the SS group only, was greater in the SS group than in the NSS group under the high-salt diet. The salt-induced suppression rate of PRA and PAC was similar between the SS and NSS groups.ConclusionBP fails to fall during the night under the high-salt diet in patients with the SS type of essential hypertension. This may be related to the lack of nocturnal decrease in sympathetic nervous activity.