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In an open-label study, glycine was administered orally (10.8 g/day in three divided doses) to six chronically psychotic patients, as an adjunct to conventional neuroleptic therapy, for periods extending from 4 days to 8 weeks. Glycine was administered in an effort to facilitate endogenous glutamatergic transmission at the level of the N-methyl-D-aspartate (NMDA) receptor complex, since a glutamatergic deficiency in the pathophysiology of schizophrenia has been postulated. Therapeutic efficacy was assessed with standardized psychiatric rating scales. Beneficial effects on behavioral symptomatology were observed in two patients, whereas two others worsened. In one of the two responders, clinical deterioration occurred after glycine withdrawal consistent with a positive adjuvant effect in this patient. However, glycine rechallenge in this patient was not associated with the clinical improvement seen during the initial glycine period. Clinical worsening was not observed after glycine discontinuation in the second responder. Glycine administration reduced neuroleptic-induced muscle stiffness and extrapyramidal dysfunction in three of the six patients. All patients tolerated the clinical trial. The limited penetrability of glycine across the blood-brain barrier is a major limitation of this approach to facilitating glutamatergic transmission at the level of the NMDA receptor complex.