The precise physiological roles for the nonapeptide Arginine Vasotocin (AVT) have not yet been clearly defined in the teleost. The contribution of the AVT neuroendocrine system to osmoregulation, the Hypothalamic-Pituitary-Interrenal-Axis (HPIA) and the spawning cycle have been investigated. Photoperiodic regulation, interaction with cortisol, UII and melatonin and a possible integrative function for the AVT system has also been considered. Monthly analysis of plasma and stored AVT concentrations has produced an annual profile for changes in the AVT system, highlighting a number of patterns. Plasma AVT concentration is higher in males than females, both plasma and stored AVT levels are higher in the winter sexually active months (Nov to Apr) in conjunction with elevated plasma osmolality. Cl-, Na+, Mg++, Ca++, GSI, cortisol and UII concomitant to decreased stored AVT and UII concentrations, in female flounder. This suggests a functional association of the AVT system with all the latter mentioned parameters and indicates roles for AVT in osmoregulation, spawning and the HPIA in female flounder. A significant peak and trough is observed in circulating AVT levels of female fish pre- and post-spawning respectively, suggesting a role for AVT in the spawning process. Plasma AVT level fluctuated on a diurnal basis, with high levels in the day and low levels at night and was inversely related to plasma melatonin measurements, suggesting an interaction between the two hormones. Plasma concentrations and stored AVT levels were measured following the acute hyper- and hypo-osmotic challenge associated with FW to SW and SW to FW transfer respectively. Plasma AVT levels fell initially following FW transfer concomitant to a rise in stored AVT levels. Acute hyperosmotic challenge produced the opposite reaction with plasma AVT levels rising concommitant to decreased stored AVT in the initial stages following SW transfer, suggesting a role for AVT in adaptation to hyperosmotic media. Positive correlations were observed between plasma AVT and osmolality and Cl- following both hypo- and hyper-osmotic challenge suggesting a functional relationship between these plasma parameters with AVT action and/ or control of AVT secretion. 6 days following both hypo- and hyper-osmotic challenge plasma and stored AVT levels were returning to control levels despite maintenance of altered osmolality and monovalent ion homeostasis suggesting that altered target tissue action and sensitivity is likely. Glucocorticoid receptors are found colocalised with AVT in the nucleus pre-opticus magnocellularis (NPOmc). Acute confinement induced a significant increase in pro-VT (AVT precursor hormone) mRNA expression in the NPOmc concomitant to an increase in plasma cortisol levels indicating a role for AVT in HPIA activation. Pro-VT mRNA levels remain elevated after plasma cortisol levels had regained control values indicating preparation of the HPIA at the hypothalamic level for additional stress. The variety of roles suggested in this study indicates interaction of the AVT system in a number of physiological and endocrine systems possibly suggesting an integrative role for this hormone.