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Background: D-Cycloserine, a partial agonist at the glycine recognition site of the NMDA receptor has previously been shown to improve negative symptoms when added to conventional antipsychotics and to worsen negative symptoms when added to clozapine. The purpose of this study was to examine the effects of D-cycloserine when added to risperidone on negative symptoms of schizophrenia., Method: Ten patients with schizophrenia who were treated with risperidone completed consecutive two week trials of placebo and four doses of D-cycloserine. Clinical assessments were videotaped and were scored by a rater who was blind to temporal sequence., Results: D-Cycloserine at a dose of 50mg/day was associated with significant reduction in negative symptoms (mean=10%). Ratings of depression, extrapyramidal side effects, and cognitive function were unchanged. Serum concentrations of glutamate and serine increased significantly on this dose of D-cycloserine., Conclusions: This preliminary study suggests that combination of D-cycloserine, 50mg/day, with risperidone may improve negative symptoms of schizophrenia over a narrow dose range. The degree of improvement appears to be intermediate between improvement of negative symptoms observed with combination of D-cycloserine with conventional antipsychotics and worsening of negative symptoms observed with combination of D-cycloserine with clozapine in previous trials of identical design.