Multicenter, randomized, double-blinded placebo controlled parallel-groups clinical trial designed to test the hypothesis that interventions (glycine or d-cycloserine) intended to increase glutamatergic activity by action at the NMDA receptor will reduce persistant negative symptoms and cognitive impairments of patients with schizophrenia or schizoaffective disorder. After an initial screening phase to establish clinical stability and eligibility, patients were assigned to one of three adjunctive treatments (placebo, d-cycloserine or glycine)for 16 weeks of double-blind treatment. Patients remained on a stable dose of antipsychotic therapy (other than clozapine) throughout the study.