Objective: To evaluate the antipsychotic effects of olanzapine. Methods: The effects of olanzapine on amphetamine induced forced action and 5-hydroxytryptophan (5-HTP) induced head twitches were observed in mice. The experiments of catalepsy induced by olanzapine were carried out in r ats. The results were compared with those of clozapine and haloperidol. Results: Olanzapine (2.4-9.6 mg/kg, ig, ED50 5.88 mg/kg), clozapin e (12-48 mg/kg, ig, ED50 37.16 mg/kg), haloperidol (0.225-0.900 mg/kg , ig, ED50 0.74 mg/kg) could significantly antagonize amphetamine-induced f orced action in mice in dose-dependent manner. Olanzapine (0.3-1.2 mg/kg, ig, ED50 0.54 mg/kg), clozapine (1.5-6.0 mg/kg, ig, ED50 3.10 mg/kg), haloperidol (0.225-0.900 mg/kg, ig, ED50 0.71 mg/kg) coul d significantly antagonize 5-HTP induced head twitches in mice. In rats cataleps y tests, the ig, ED50 of olanzapine, clozapine, haloperidol were 26, 149, 0.54 mg/kg, respectively. Conclusion: Olanzapine possesses dopa mine and 5-HT antagonist activity in vivo, and was a more potent 5-HT anta gonist than dopamine antagonist. Olanzapine has a low property for producing ex trapyramidal symptoms. Olanzapine is expected to find a place as a valuable alt ernative in classical antipsychotics. The combination of olanzapine and haloper idol may be a good recipe for some patients.