Drinking response to the intravenous administration of insulin (0.1 U/kg) was studied in 15 volunteers (eight males and seven females). Water intake was significantly higher after insulin than after saline administration during the 90-min period studied. Plasma glucose decreased significantly in individuals receiving insulin and the time of the maximum decrease (30 min) was concurrent with the beginning of water intake. Haematocrit values in the insulin-treated group were also significantly higher at that time. Plasma renin activity (PRA) after insulin administration was higher than under basal conditions or after saline injection. On the other hand, psychological responses indicated that insulin probably elicits thirst prior to the hunger which appears with hypoglycaemia. A possible role of endogenous insulin in meal-related thirst is hypothesized.