Adult male frogs (Rana ridibunda) were subjected to several volumetric and osmometric stimuli and the influence on circulating concentrations of arginine vasotocin (AVT) and mesotocin was studied by the use of highly specific radioimmunoassays. During progressive blood withdrawal (haemorrhage group) urine flow decreased to zero, whereas no change occurred in the plasma and urine osmolality. Control levels of 34.3 +/- 7.3 pmol AVT/1 gradually increased up to 638.3 +/- 179.1 pmol/1 (P less than 0.001) after a blood loss of up to 50-60% of the blood volume. Plasma mesotocin concentrations also increased from 42.4 +/- 9.2 to 70.8 +/- 12.0 pmol/1 (n = 7). Hypervolaemia, produced by the repeated intravenous injection of isotonic Ringer solution, increased the urine flow and osmolality compared to controls but had no influence on the plasma levels of AVT and mesotocin. Hypernatraemia without volume change profoundly increased the urine osmolality but the urine flow was not affected; the plasma concentrations of AVT and mesotocin remained at the control level. Finally, during a 1-h immobilization stress a pronounced antidiuresis occurred in the presence of a constant plasma and urine osmolality and control plasma levels of AVT and mesotocin. It is concluded that the release of AVT and, to a smaller extent, of mesotocin is under volumetric control.