A hypofunction of the glutamatergic system and NMDA receptors in schizophrenia has been hypothesized. Therefore, stimulation of these receptors could be of benefit to patients with schizophrenia. D-cycloserine has been used for this purpose. This study reports the effects of 100 mg D-cycloserine, when added to typical antipsychotics in chronic schizophrenic patients exhibiting prominent negative symptoms, using a placebo-controlled, double-blind, parallel, design. D-cycloserine (Paschen 1996) and schizophrenia (Javitt and Zukin 1991;Henn 1995;Olney and Farber 1995).In schizophrenia, a role for excitatory amino acids and NMDA receptors was suggested when NMDA receptor antagonists, such as phencyclidine (PCP) and ketamine, were found to induce symptoms resembling the positive and negative symptoms of schizophrenia in healthy subjects and exacerbate such symptoms in patients with schizophrenia (Luby et al. 1962;Javitt and Zukin 1991;Krystal et al. 1994;Lahti et al. 1995;Malhotra et al. 1997b). A hypofunction of the glutamatergic system and NMDA receptors has been hypothesized in schizophrenia (for review, see Olney and Farber 1995) and appears to be supported by post-mortem studies (Nishikawa et al. 1983;Toru et al. 1988;Ishimaru et al. 1994;Tsai et al. 1995). If schizophrenic symptoms are the result of the diminished functioning of NMDA receptors, stimulation of these receptors could be beneficial for patients with schizophrenia. Glycine and D-cycloserine stimulate the strychnine-insensitive glycine recognition site of the NMDA receptor and, by inference, may ameliorate schizophrenic symptoms. Some studies have used glycine for this purpose. In a double-blind, parallel, placebo-controlled study with seven patients in each group, glycine treatment during 8 weeks in a dose ف 30 grams daily, added to typical antipsychotics, selectively reduced the scores on the negative symptom subscale of the Positive and Negative Symptoms Scale (PANSS) (Javitt et al. 1994). A similar result has also been reported in 11 schizophrenic patients using glycine in dosages between 40 and 80 grams daily in a doubleblind crossover design with treatment periods of 6 weeks (Heresco-Levy et al. 1996). These two studies suggest that stimulation of the NMDA receptor may ameliorate the negative symptoms of schizophrenia.As a treatment of schizophrenic symptoms, glycine's major limitation is its limited access to the central nervous system (CNS). Contrary to glycine, the partial NMDA agonist D-cycloserine, an antibiotic used in the treatment of tuberculosis (Walker and Murdoch 1957), rapidly passes the blood-brain barrier and may therefore be more suitable for the treatment of patients with schizophrenia than glycine. However, D-cycloserine acts as a partial agonist of the NMDA receptor, with around 60%-90% of the agonistic efficacy of glycine (Chessell et al. 1991;Priestley et al. 1995) and with between 6 to 20 times less affinity for the glycine recognition site of the NMDA receptor than glycine (Ishimaru et al. 1994;Priestley et al. 1995)...