The effect of bupropion, a new clinically active antidepressant drug, administered repeatedly to rats and mice, was compared with that of imipramine. The drugs were given orally twice a day for 14 days, bupropion in a dose of 20 mg/kg, imipramine--10 mg/kg. The action of bupropion was studied in the following tests: clonidine-induced aggression in mice, open field in rats, amphetamine-induced hypermotility in rats, clenbuterol-induced hyperthermia in rats kept at high ambient temperature. Moreover, the effect of bupropion on 3H-prazosin binding to membranes from the cerebral cortex was measured. On the basis of the results obtained it may be concluded that in some tests chronically administered bupropion acts like imipramine; as both drugs potentiate the amphetamine-enhanced locomotor activity, attenuate the hyperthermic response to clenbuterol and increase the number of 3H-prazosin binding sites in the cerebral cortex.