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Hydration status and the effects of hypohydration have been the topic of much public and scientific debate in recent years. While many physiological responses to hypohydration have been studied extensively, the subjective responses to hypohydration have largely been ignored. The present investigation was designed to investigate the physiological responses and subjective feelings resulting from 13, 24 and 37h of fluid restriction (FR) and to compare these with a euhydration (EU) trial of the same duration in fifteen healthy volunteers. The volunteers were nine men and six women of mean age 30 (sd 12) years and body mass 71·5 (sd 13·4)kg. Urine and blood samples were collected and subjective feelings recorded on a 100mm verbally anchored questionnaire at intervals throughout the investigation. In the EU trial the subjects maintained their normal diet. Body mass decreased by 2·7 (sd 0·6)% at 37h in the FR trial and did not change significantly in the EU trial. Food intake in the FR trial (n 10) provided an estimated water intake of 487 (sd 335)ml and urinary losses (n 15) amounted to 1·37 (sd 0·39) litres. This is in comparison with an estimated water intake of 3168 (sd 1167)ml and a urinary loss of 2·76 (sd 1·11) litres in the EU trial. Plasma osmolality and angiotensin II concentrations increased from 0–37h with FR. Plasma volume decreased linearly throughout the FR trial amounting to a 6·2 (sd 5·1)% reduction by 37h. Thirst increased from 0–13h of FR then did not increase further (P>0·05). The subjects reported feelings of headache during the FR trial and also that their ability to concentrate and their alertness were reduced.