• 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 ]

The study aims to assess the effects of single dose and repeated weekly dosing of 50mg d-cycloserine versus placebo on cognitive and memory functioning in schizophrenia patients. The study will also examine the effects of 50mg d-cycloserine on positive symptoms and negative symptoms, as well as assess tolerability and side-effects.

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.

Inclusion Criteria:

1. Male or female
2. Age 18-65 years
3. Diagnosis of schizophrenia or schizoaffective disorder, depressed type
4. Stable dose of antipsychotic for at least 4 weeks.
5. Able to provide informed consent
6. Able to complete a cognitive battery

Exclusion Criteria:

1. Current treatment with clozapine
2. Dementia
3. Seizure disorder
4. Unstable medical illness
5. Active substance abuse
6. Pregnancy, nursing, or unwilling to use appropriate birth control measures during participation if female and fertile.