Alpha-2 adrenoceptor antagonists remove the presynaptic negative feed-back control of norepinephrine release, by this enhancing the naturally occurring adrenergic activation. Male Long-Evans rats were injected with 1 mg/kg CH-38083, an alpha 2 adrenoceptor blocker. Twenty minutes later a size matched Wistar or Long-Evans opponent, respectively, was introduced in their home cage for 15 min (the Wistar strain is more aggressive than the Long-Evans in these circumstances). The treatment increased the number of biting attacks several folds in both situations. The correlations existing between the behavior of the intruder and the aggressiveness of the resident (treated) rat led to the conclusion that the catecholaminergic treatment increased the sensitivity of the animals to the behavioral actions of the intruder. The treated animals responded in an exaggerated fashion compared to their saline treated counterparts. An increase in the corticosterone response of the treated rats paralleled the increase in attack frequency, and seemed not to depend on the challenge received from the intruder.