A specific radioimmunoassay was developed and characterised for the measurement of arginine vasotocin (A VT) in teleost fish. Crossreactivity of the antibody with the other teleost neurohypophysial peptide, isotocin, was less than 1 % and the sensitivity of the assay 0.25 pglassay tube. Rapid blood sampling, within 1 minute of capture, by needle puncture of the caudal blood vessels, and collection of blood through chronic arterial cannula 1 - 4 days after surgery were identified as procedures which did not influence plasma ion levels, osmolality or hormone concentrations. These procedures were used in later experimental protocols examining the effects of chronic and acute osmotic stimuli on plasma A VT concentrations. Plasma A VT concentrations in a range of euryhaline and stenohaline teleost fish were measured at 10-12 to low 10-11 M (1-20 pglml). There was no consistent difference in circulating A VT levels in euryhaline fish (flounder, eel and trout) adapted to fresh water (FW) or seawater (SW) for at least 2 weeks. The flounder was studied in greater detail and seasonal influences were identified as having an effected on osmoregulation. In the winter/spring period plasma osmolality was significantly lower in FW adapted fish than at other times of the year. In SW adapted fish at this time plasma osmolality was elevated and this was accompanied by elevated plasma A VT concentrations. The winter/spring period was associated with the development of the gonads and this factor may affect osmoregulation in the flounder. In SW adapted flounder a positive linear relationship between plasma A VT and plasma osmolality and ion levels was observed, indicating a link between these two parameters but no such relationship was evident in FW adapted fish. Chronically cannulated SW adapted flounder were used to examine the effects of acute osmotic stimuli on plasma A VT concentrations. Intraperitoneal injection of hypertonic saline acutely increased plasma osmolality and was associated with a significant increase in plasma A VT concentration, indicating that A VT secretion was sensitive to increased plasma osmolality. The expansion of extracellar fluid volume via intravascular infusion of isotonic saline reduced plasma A VT concentrations independently of any change in plasma osmolality. This indicated that vascular volume also influences plasma A VT concentrations. Reducing blood volume by haemorrhage and lowering plasma osmolality by infusion of hypotonic saline did not result in any change in plasma A VT concentrations. Plasma A VT concentrations were measured in lamprey at a 10-12 M concentration, and the transfer of lamprey from FW to Baltic SW caused an initial rise in plasma A VT concentrations, suggesting that A VT is involved in osmoregulation in these fish. A VT immunoreactivity was also demonstrated in two species of elasmobranch fish in a 10-12 to low 10-11 M range.