The neuropeptides arginine-vasotocin (AVT) and corticotropin-releasing hormone (CRH) are involved in the hypothalamic-pituitary-interrenal (HPI) axis. During stress, the HPI axis is activated and cortisol is released into the blood. In addition to their role in the HPI axis, AVT and CRH also have behavioural effects. The roles of AVT and CRH in stress responses and agonistic behaviour were studied in this thesis, using two different models. In the first model, two strains of rainbow trout (Onchorhynchus mykiss) divergent in stress-induced release of cortisol were investigated. This was done by observing behaviour and stress responses under different conditions. These strains were found to have divergent stress coping strategies based on the observed behaviour and levels of plasma cortisol. This divergence in behaviour could be associated with the CRH system, since the mRNA levels of CRH differed between the strains during stress. However, no differences between strains were observed in AVT or its receptor expressions. In the second model, non-selected rainbow trout were paired and the effect of intracerebroventricular (icv) injections of an active substance (AVT, CRH or the CRH related peptide Urotensin-I (UI)) on fights for dominance was investigated. One fish of the pair received the active substance icv and the other received saline icv. Fish receiving AVT became subordinate in accordance with the suggestion that AVT attenuates aggression in territorial vertebrates. Fish receiving CRH became subordinate whereas UI showed no effect on fights for dominance. Further, both CRH and UI induced an anxiety-related behaviour similar to non-ambulatory motor activity in rats. In addition, CRH appeared to affect the dopaminergic and serotonergic systems. In this thesis, it is suggested that CRH is involved in the behavioural modulation of the stress coping strategies in teleost fish. Further, AVT and CRH seem to act inhibitory on aggressive behaviour.