Imidazole (IMI) (from 18.7 to 300 mg/Kg) i.p. injected in adult rats induced shaking, which was antagonized by both morphine (MOR) and haloperidol (HALO) but not by methysergide (MET). I.p. IMI pretreatment inhibited the penile erections (PE) and stretching and yawning (SY) typically elicited by N-n-propylnorapomorphine (NPA), a well-known CNS dopamine (DA) receptor stimulant, injected either i.p. or i.c.v., whereas it enhanced stereotyped behavior (SB). IMI had similar effects on the same parameters considered when injected before lisuride, an ergot derivative also active as a central DA receptor agonist. In this case not only SB but also and above all aggressiveness were markedly potentiated, both the signs appearing at doses of lisuride which were "per se" ineffective. Aggressiveness, like SB, was not sex linked and was antagonized by HALO and MOR, but not by MET. IMI alone potentiated the fighting induced by electrical shock, an effect which was abolished by HALO pretreatment. Considering the results obtained as a whole it is submitted that IMI antagonizes PE and SY through a selective blockade of a class of DA receptors, presumably DA presynaptic autoinhibitors, thus potentiating SB and aggressiveness, which involve stimulation of DA postsynaptic receptors.