To determine the effect of hydration on the early osmotic and intravascular volume and endocrine responses to water immersion the hematocrit, hemoglobin, plasma renin activity (PRA), and plasma electrolyte, aldosterone (PA), and vasopressin (PVP) concentrations were measured during immersion following 24-h dehydration; these were compared with corresponding values following rapid rehydration. Six men and one woman (age 23-46 yr) underwent 45 min of standing immersion to the neck preceded by 45-min standing without immersion, first dehydrated, and then 105 min later after rehydration with water. Immersion caused an isotonic expansion of the plasma volume (P less than 0.001), which occurred independently of hydration status. Suppression of PRA (P less than 0.001) and PA (P less than 0.001) during both immersions also occurred independently of hydration status. Suppression of plasma vasopressin was observed during dehydrated immersion (P less than 0.001) but not during rehydrated immersion. It is concluded that plasma tonicity is not a factor influencing PVP suppression during water immersion.