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To investigate precisely the fluid shifts associated with water drinking in humans, we measured continuously blood density and plasma electrolyte concentrations using the mechanical oscillator technique and ion-selective electrodes, respectively, in healthy young volunteers before (10 min) and after (48 min) water drinking for a period of 2 min. Beat-by-beat blood pressure was also monitored throughout the experiment. Drinking 1 l tap water caused a transient increase in blood density immediately after the drinking episode (from 1051.1±0.5 g/l before drinking to 1051.8±0.5 g/l 4 min after the start of drinking, P<0.05), followed by a gradual reduction (1050.1±0.5 g/l at 31 min). This drinking-induced change paralleled those of haematocrit, plasma density and plasma volume. Plasma [Na+] and [Cl–] and osmolality decreased after drinking without transient increases and reached minima at about 30 min. A transient increase in mean arterial blood pressure was observed prior to the increase in blood density. These findings suggest that water drinking causes a biphasic change in plasma volume: initial haemoconcentration, probably due to sympathetic acceleration, followed by haemodilution due to the post-absorptive effect, and further suggest that the fluid shift associated with the initial haemoconcentration is isosmotic.