The aim of this study was to replicate our earlier finding of elevated angiotensin-converting enzyme (ACE) in cerebrospinal fluid (CSF) in schizophrenia and to elucidate the role of neuroleptic treatment in this phenomenon. Drug-free and medicated patients with acute schizophrenic psychoses, as well as healthy controls were recruited. Levels of ACE were measured in CSF and serum from 7 drug-free patients, 36 neuroleptic-treated patients, and 19 healthy control subjects. Although ACE levels in CSF did not differ between patients and controls, the drug-free patients showed significantly lower levels than the neuroleptic-treated patients. Serum ACE did not differ between groups. The elevation of CSF ACE may be more prominent in patients with deficit symptoms than in those with mainly psychotic symptoms. The possible enhancement of CSF ACE production or solubility by neuroleptic treatment is discussed. Elevated ACE levels in CSF may, together with other possible factors, cause polydipsia, stimulate secretion of arginine vasopressin, and even affect neuron growth and differentiation in schizophrenic psychoses.