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In experiments on 8 healthy young male volunteers, the ingestion of a large meal was found to cause plasma osmolality to rise from 288.8 +/- 0.8 (mean +/- s.e. mean) to 295.6 +/- 0.9 mmol/kg at 4 hours (P less than 0.001). There was an accompanying rise in plasma sodium (Na) from 141.9 +/- 0.8 to 144.6 +/- 0.8 mmol/l, also at 4 hours (P less than 0.01), but little change in other plasma electrolytes. Serum total amino acids rose slightly, non-esterified fatty acid fell minimally and changes in blood glucose concentrations were unremarkable. Thirst was experienced at plasma osmolality of 294.8 +/- 0.7 mmol/kg. Repeating the experiment either without food, or with the salt content of the meal only, was without effect on plasma Na, other solutes or osmolality. Postprandial hypersomolality and hypernatraemia is probably due to movement of water from the vascular compartment to the gut, or into cells. Plasma osmolality is best measured in the fasting state.