The purpose of this study is to evaluate the effects of a four-week trial of CX516 900mg tid compared to placebo upon verbal memory, attention and negative symptoms. The AMPA receptor positive modulator, CX516, will be added to a stable dose of clozapine, olanzapine or risperidone in inpatients and outpatients with schizophrenia. The trial is intended to extend and replicate results from our previous placebo-controlled pilot trial of CX516 added to clozapine in which the investigators found improvement in memory and attention (moderate-to-large between group effect sizes) and did not observe serious side effects. Because the investigators' pilot trial also detected at two-week follow-up persistence of cognitive benefits and emergence of a large therapeutic effect upon negative symptoms, this trial will also repeat clinical and cognitive assessments at follow-up, four weeks after completion of the study medication to evaluate persistence and/or strengthening of effects.

Inclusion Criteria:

1. Diagnosis of Schizophrenia, any subtype
2. Ages 18-65 years
3. Capable of providing informed consent
4. Stable dose of clozapine, olanzapine or risperidone for at least 6 months

Exclusion Criteria:

1. Serious medical or neurological illness (unstable cardiac disease, seizure disorder, malignancy, liver or renal impairment, etc.)
2. Current substance abuse
3. Pregnancy, nursing, or unwilling to use appropriate birth control measures during participation if female and fertile
4. Unable to complete neuropsychological tests
5. History of serious blood dyscrasia requiring discontinuation of clozapine
6. Serious suicidal or homicidal risk within the past six months