Inadequate fluid intake is though to be one frequent cause of chronic constipation, although this has never been shown. In order to test whether fluid deprivation has an influence on colonic function, we studied eight healthy male volunteers (median age 23 y, range 21-28 y) with respect to stool frequency, stool weight and mean oroanal transit of radiopaque markers in a control week with a fluid intake of more than 2500 ml beverages per day and in a week with a fluid intake of less than 500 ml beverages per day. The two weeks followed each other in randomised order (with a wash out week in-between) and were standardised as to nutrition and physical activity. Stool frequency diminished from 6.9 +/- 0.9 to 4.9 +/- 0.3 (mean +/- SEM) defaecations per week (p = 0.041) and stool weight from 1.29 +/- 0.20 to 0.94 +/- 0.17 kg per week (p = 0.048) during fluid restriction. Mean oroanal transit times were similar in the two weeks. In conclusion, our study shows that a relatively short period of fluid deprivation decreases stool frequency and stool weight in healthy volunteers. Therefore, a low fluid intake may well be an aetiologic factor for chronic constipation in some patients and too low a fluid intake should be corrected. There is no evidence, however, that an increase of fluid intake within feasible limits has a beneficial effect on chronic constipation.