These experiments show that combined infusions of corticotrophin-releasing factor (CRF) and arginine vasopressin (AVP) into either the lateral ventricle or the amygdalae have synergistic effects on aggressive, investigative and other behaviours occurring during social interaction between male rats. They suggest, therefore, that the two peptides interact at intracerebral sites to control behaviour much as they do on the anterior pituitary to regulate ACTH release. CRF or AVP, alone or in combination, were infused into either the lateral ventricle (dose range: 10-250 pmol) or bilaterally into the amygdalae (dose range: 1-150 pmol) of male rats in two experiments. The rat was then paired with another, strange, male for 10 min. There was a U-shaped effect on aggressive behaviour after intra-amygdala infusions of CRF, lower doses increasing agonistic behaviour, higher ones decreasing it. This was not seen after icv infusions. AVP had no effect by either route; however, given together with CRF it potentiated the latter's effect on aggressive behaviour. Investigative behaviour was decreased by icv CRF but the effects of amygdala infusions were small. AVP had no consistent effect by either route. Combined infusions of both peptides given either icv or into the amygdala decreased investigative behaviour. Self-grooming increased, though in an irregular fashion, after incremental doses of either CRF or AVP given by either route. Both peptides given together showed additive effects on self-grooming. Climbing behaviour was lowered by CRF more prominently than by AVP and, again, the two peptides together profoundly reduced this behaviour. These experiments show that the behavioural effects of CRF and AVP on social interaction have different profiles, and that the effects of each peptide differ when it is given into the ventricles or directly into the amygdala. There is also clear evidence for synergistic effects of the two peptides on behavior after infusion by either route.