 16 in- or outpatients with DSM-IV-TR schizophrenia or schizoaffective disorder and prominent negative symptoms will be recruited for this study. This study will consist of a randomized trial of D-serine 60 mgkgd vs. placebo using a crossover design with a 2-week baseline lead-in, and two 6-week intervention arms separated by a two week, placebo controlled washout period. Biomarkers will be assessed at baseline for each treatment arm, acutely day 7 following treatment initiation, and following 6 weeks of treatment 6 biomarker sessions total. Primary biomarker outcome measures will include 1 amplitude of the mismatch negativity MMN waveform and 2 amplitude of the visual P1 potential. Symptomatic outcome measures will include PANSS and composite score of the MATRICS neuropsychological battery. The study will be supported from ongoing NIMH-funded Cooperative Drug Development Grant CDDG to the PI. 