Plasma adrenocorticotropin hormone and cortisol responses to apmorphine are significantly blunted in patients with schizophrenia (SCH) compared to normal controls. Plasma homovanillic acid levels are positively correlated with basal cortisol levels and inversely correlated with APO-stimulated ACTH and cortisol response in SCH. Chronicc treatment with clozapine and olanzapine enhances the plasma ACTH and cortisol responses to APO and decreases pHVA levels in SCH. The change in the plasma ACTH and cortisol responses to APO or the decrease in pHVA levels, or both, after treatment with clozapine or olanzapine predicts clinical response to clozapine or olanzapine.