Duodenal hypertonicity can induce satiety, but whether duodenal osmolality varies during eating enough to influence meal size is not clear. To find out more about this, duodenal samples were taken via implanted catheters from 10 young pigs under three feeding conditions. 1) The pigs were fasted 4-5 h and then given a meal with drinking water available. Duodenal osmolality rose from a premeal level of 297 mosmol/kgH2O to a peak of 430 mosmol/kgH2O at 35 min after the meal began and then returned to the premeal level after 2.2 h. 2) When pigs were similarly fasted and then given a meal, but with no drinking water available during and after the meal, osmolality rose to 418 mosmol/kgH2O after 35 min and only returned to premeal levels after 4.2 h. 3) Finally, when pigs were allowed to eat and drink spontaneously, osmolality rose from a mean of 281 to only 308 mosmol/kgH2O after meals, in part because of water drinking in association with meals and in part because the spontaneous meals were smaller. The results demonstrate that duodenal osmolality can rise significantly after meals but that hyperosmolality is moderated by water drinking.