The proposed study is a multicenter, randomized, double blind comparison of MK-0777 GEM 3 mg BID, MK-0777 GEM 8 mg BID, and placebo. The total sample will consist of 90 clinically stable patients with DSM IV TR schizophrenia, with 30 subjects randomized to each group. A best estimate diagnostic approach will be utilized, in which information from the Structured Clinical Interview for DSM-IV (Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition) (First et al, 1997) is supplemented by information from family informants, previous psychiatrists, and medical records to generate a diagnosis. The projected number of subjects to be recruited from each site is 12-13. There will be a 2 week, placebo lead-in evaluation phase, in which subjects will undergo baseline diagnostic; medical, including a physical examination, EKG (electrocardiogram), CBC (Complete Blood Count), complete metabolic panel, urine toxicology, and UA (urinalysis); psychiatric; and neurocognitive, symptom level and functional capacity and patient self-report of cognitive function assessments. In addition, all subjects will receive a slit-lamp eye examination. At the end of the evaluation phase, subjects will be randomized to one of two MK-0777 doses or placebo. The double-blind treatment phase will be 4 weeks. Subjects will receive bi-weekly symptom assessments and weekly side effect and vital sign assessments. At week 4, subjects will undergo repeat administration of the neuropsychological test battery and the functional capacity and patient self-report of cognitive function measures. These assessments will be done over a two-day period. Subjects will have blood samples collected for antipsychotic and MK-0777 levels at week 4. An EKG (electrocardiogram) will be obtained at the end of the double-blind study. Slit-lamp eye examinations will be conducted at study completion, 6 months and 12 months after study completion. After the completion of the 4-week double-blind phase, there will be a 4-day follow-up phase during which subjects will be tapered off study medication.