This study examined the putative inhibitory effect of the atypical antipsychotic, olanzapine, on dizocilpine (MK-801)-induced stereotypy and hyperlocomotion. Dizocilpine (0.1, 0.25 and 0.5 mg/kg) produced a dose-dependent increase in both stereotypy and hyperlocomotion. Pretreatment with olanzapine (0.25 and 0.5 mg/kg) inhibited the dizocilpine (0.5 mg/kg)-induced hyperlocomotion but not the stereotypy. At the higher doses (1, 2 and 4 mg/kg), olanzapine blocked both the stereotypy and hyperlocomotion induced by dizocilpine. Similarly, olanzapine, 0.25 and 0.5 mg/kg, did not inhibit apomorphine (3 mg/kg)-induced stereotypy, whereas the higher dose (1 mg/kg) blocked it. We also studied the effect of olanzapine on spontaneous locomotor activity and catalepsy. Olanzapine (0.25 and 0.5 mg/kg) did not induce a decrease in spontaneous locomotor activity but did so at the higher doses (1, 2 and 4 mg/kg). The lower doses (0.25, 0.5 and 1 mg/kg) did not induce catalepsy but higher doses (2 and 4 mg/kg) induced a significant catalepsy which lasted for more than 4 h. The results thus showed that, at lower doses, olanzapine selectively inhibited behaviours mediated by the mesolimbic/mesocortical system while at higher doses it inhibited behaviours mediated by both mesolimbic/mesocortical and nigrostriatal systems. Therefore, the minimal extrapyramidal side-effects produced by olanzapine at effective doses might be due to its preferential action at the mesolimbic/mesocortical area.