• Main Outcome Measure: The change from baseline to week 8 on the SANS total score and on a composite cognitive score. [ Time Frame: 8 Weeks ] [ Designated as safety issue: No ]
  

• comparisons of treatment effects on the Positive Syndrome Subscale of the PANSS and CGI global improvement score at week 8 [ Time Frame: 8 Weeks ] [ Designated as safety issue: No ]
  
• change from baseline at the first repeat administration of the Logical Memory Test (immediate recall tested at week 1 and delayed recall tested at week 2) [ Time Frame: 1 week ] [ Designated as safety issue: No ]
  

This is a ten-week, parallel-group, placebo-controlled trial examining the cognitive effects at weeks 1, 2, 3. 4, 5, 6, 7, 8 & 10 of once-weekly oral D-cycloserine 50 mg added to a stable dose of antipsychotic for 8 weeks in 60 adult outpatients with schizophrenia.

Specific aims:

1. Assess the effects of a single dose of D-cycloserine 50 mg on cognitive functioning compared to placebo.
2. Assess the effects of repeated weekly dosing of D-cycloserine on cognitive functioning at week 8 compared to placebo.
3. Assess the effects of repeated weekly dosing of D-cycloserine on memory functioning once a week 1 hour after medication administration compared to placebo.
4. Assess the persistence of learned information in a no-treatment follow-up assessment at Week 10 in the D-cycloserine group compared to the placebo group.
5. Assess effects of weekly D-cycloserine dosing on positive & negative symptoms at week 8 compared to placebo.
6. Assess tolerability and side effects of weekly D-cycloserine compared to placebo.
7. Assess the effects of d-cycloserine dosed weekly for seven weeks on reward responsiveness as measured with the response bias task compared with placebo.
8. Assess the effects of d-cycloserine dosed weekly for seven weeks on measures of functioning.