The effects of two-day water deprivation and hyperhydration (provision of 4% sucrose solution for 48 h) on levels of serotonin and its major metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the midbrain and hypothalamus were studied in Wistar rats. The rates of diuresis (0.05 +/- 0.01 and 0.84 +/- 0.12 ml/h/100 g in water deprivation and hyperhydration respectively) and urine osmolality (1896 +/- 182 and 50 +/- 13 mOsm/kg) reflected increases and decreases in blood vasopressin levels. Water deprivation was associated with a significant increase in 5-HIAA levels in the midbrain and hypothalamus, along with a decrease in serotonin levels and a three-fold increase in serotonin catabolism (the 5-HIAA:serotonin concentration ratio). Hyperhydration induced moderate increases in serotonin and 5-HIAA levels in the hypothalamus with no changes in the midbrain. The blood corticosterone level doubled in water deprivation and decreased in hyperhydration. It is suggested that activation of the serotoninergic system induces a complex adaptive reaction in water deprivation. including mechanisms specific for the regulation of water-electrolyte homeostasis and non-specific stress mechanisms (vasopressin and corticoliberin secretion).