Behavioural effects of typical and atypical antipsychotic agents were compared in unrestrained rhesus monkey (Macaca mulatta) living in social colonies. The behaviours were categorized as social, solitary and abnormal. They were studied with the help of video cameras fixed in the observation chamber. The behavioural effects were recorded on videotape and analyzed for significant changes. Chlorpromazine (2.5-10 mg/kg, i.m.), haloperidol (0.01-0.04 mg/kg, i.m.), risperidone (0.05-0.2 mg/kg, p.o.) and clozapine (5-20 mg/kg, p.o.) induced significant alterations in parameters of social and solitary behaviour. Chlorpromazine produced a marked decrease in locomotor activity whereas haloperidol showed marked extrapyramidal effects. Risperidone produced minimal extrapyramidal effects and sedation compared to haloperidol and chlorpromazine. Clozapine had intermediate extrapyramidal effects similar to those of chlorpromazine but it produced hypersalivation and dose-related sedation. Thus, risperidone had advantages over the other antipsychotics used in this study because it did not produce salivation, had minimal extrapyramidal effects and caused less sedation. These antipsychotic drugs produced many behavioural effects in the rhesus monkey that were similar to their clinically observed effects. A study of behavioural effects in the monkey can thus be a useful predictive tool in the preclinical development of new antipsychotics.