The vasopressin (VP) and aldosterone responses of swamp buffaloes and Friesian cattle were compared during 3 days of ad libitum water availability and 3 days of water deprivation. During the period of hydration, buffaloes showed significantly higher VP concentrations (2.31±0.09 v. 1.23±0.07 pg/ml), in spite of larger water excretion rate through urine, relative to cattle. During the period of dehydration, although VP increased progressively in both species, the concentration in the buffaloes (66.7±18.6 pg/ml) was five times higher than that of the cattle (12.5±3.2 pg/ml) on Day 3. Even though the rate of urine excretion in the buffaloes decreased markedly during water deprivation, the total volume of urine excretion was still greater than that of the cattle. Haematocrit, serum osmotic pressure and total protein in both species increased significantly during water deprivation, but the rate of the increase was greater in buffaloes than in cattle. There was no correlation between sodium or potassium excretion and aldosterone concentration in either species. These results demonstrated that the VP elevation in buffaloes was largely attributable to the greater decrease of plasma volume caused by the greater water excretion. The lower retentivity of plasma volume probably means that buffaloes have a greater dependence on external water rather than to conserve internal water, because of the evolutionary adaptation of buffaloes to wet environments.