To clarify the physiological roles of the renin-angiotensin-aldosterone system (RAAS) and arginine vasotocin (AVT) on body fluid regulation in amphibians, we measured plasma concentrations of aldosterone (ALDO), angiotensin II (ANG II), and AVT after various osmotic challenges in the marine toad, Bufo marinus (Bufonidae). Hematocrit value (Ht) as an indicator of plasma volume, plasma osmolality and concentrations of plasma components (Na(+), Cl(-), K(+), and urea) were also measured. The toads were maintained under various osmotic treatments for 7 days. In dehydrated toads, plasma concentrations of ALDO, ANG II, AVT, and all plasma components measured were increased. In toads maintained in 300 mosmol/kg H(2)O NaCl solution, plasma osmolality, Na(+), Cl(-), urea, and plasma AVT concentrations were significantly increased, and Ht and plasma concentrations of ALDO and ANG II were significantly decreased. In toads maintained in tap water, plasma osmolality, and concentrations of Na(+) and ALDO were significantly decreased. We also estimated total body water (TBW), plasma volume (PV) using Evans Blue dye and Ht in the toads under various osmotic treatments. In dehydrated toads, TBW and PV were significantly decreased and Ht was significantly increased in comparison with those of control. In toads maintained in 300 mosmol/kg H(2)O NaCl solution, TBW and PV were significantly increased and Ht was significantly decreased in comparison with those of control. There was a significant negative correlation between Ht and PV or TBW. These results show that dehydration, which induces hypovolemic and hyperosmotic conditions, stimulates increases of plasma ALDO, ANG II, and AVT concentrations, while hypervolemic treatment induces decreases of plasma ALDO and ANG II concentrations. There were significant correlations between plasma osmolality and AVT concentration, between Ht and concentrations of RAAS hormones, and between plasma concentrations of ALDO and ANG II. These results suggest that volumetric and osmometric systems regulated by RAAS hormones and AVT are present in B. marinus.