Platelet 5-HT and plasma cortisol concentrations were determined in 59 schizophrenic patients with different time course of illness before and after dexamethasone suppression test (DST). An abnormal DST (nonsuppression) was observed in 51% of patients. In these patients basal cortisol and platelet 5-HT concentrations were higher than in patients with normal DST. After DST, plasma cortisol levels were higher in nonsuppressors with intermittent and intermittent-chronic time course, whereas platelet 5-HT concentrations were increased in nonsuppressors with intermittent-chronic time course. The results suggest that schizophrenic patients have dysregulated hypothalamic-pituitary-adrenal axis as shown by a high rate of DST nonsuppression, and that nonsuppressors showed hypercortisolemia and hyperserotonemia independent of the time course of schizophrenia. No significant association between DST and time course of the illness was found.