It has previously been found that a number of typical and atypical antidepressants, given chronically, intensify clonidine-induced aggressiveness in mice. Further experiments now show that chronic, but not acute, administration of nisoxetine, a selective inhibitor of noradrenaline uptake, potentiates clonidine-induced aggressiveness. Citalopram and fluvoxamine, two selective inhibitors of serotonin uptake, have no such action. Of the two isomers of flupenthixol, only the trans-form potentiates clonidine-induced aggressiveness of chronic experiments. The cis-form induces an inhibiting effect. Clonidine-induced aggressiveness is also intensified by chronic, but not by acute, administration of pizotifen, an antagonist fo serotonin and noradrenaline. The results seem to support the previous hypothesis that potentiation of clonidine-induced aggressiveness is mediated by an alpha 1-adrenergic mechanism.