Following an initial rise in locomotor activity, apomorphine in large doses causes a concurrent rise in brain serotonin levels, locomotor akinesia, and stereotypic gnawing. However, reports to date have failed to observe any effect of pretreatment with serotonin depletors parachlorophenylalanine (pCPA) or parachloroamphetamine (pCA) on apomorphine-induced stereotypy. In the present study the effects of pCPA (250 or 400 mg/kg i.p., 3 days) and pCA (6.4 or 10.4 mg/kg i.p., 3 days) pretreatment on apomorphine-induced (5.0 mg/kg s.c., 5 min) behavior of male rats in the open-field were compared. For half of the trials in the 78 min session, the rats were alone and for half of the trials they were paired with an untreated male rat. pCA pretreatment increased the frequency of line-crossing and of jumping, whereas pCPA pretreatment increased the duration of bouts of locomotion and gnawing. These behavioral differences may be related to the interaction of pCA and pCPA with dopaminergic subsystems in the brain.