Platelet-rich plasma from healthy controls was pre-treated with neuroleptics of the phenothiazine, butyrophenone or benzamide variety before aggregation with one of the following agonist agents: ADP, adrenaline, 5-HT, collagen, platelet activating factor or ristocetin. All compounds effective as antipsychotics, except sulpiride, depressed aggregation. Unmedicated schizophrenics showed aggregation responses indistinguishable from healthy controls. However, within days of treatment with either trifluoroperazine or haloperidol responses became abnormal in acutely psychotic patients. Increased responses to 5-HT and depressed responses to platelet activating factor were detected. After 4 weeks of treatment responses tended to return to normal. Aggregation responses were normal in those patients on long-term depot neuroleptics.