This study will evaluate whether talnetant (200mg, 400mg, 600mg) twice a day is effective compared to placebo and risperidone 1-3mg twice a day in treating the positive and negative symptoms of schizophrenia. Study subjects will be treated with study drug for up to 6 weeks and will then return for a follow-up visit 2 weeks later. About 275 patients with schizophrenia will be enrolled in this study.

Inclusion criteria:

• Subjects must have schizophrenia which is not secondary to another medical condition or substance abuse.
• Require inpatient hospitalization.
• Woman may enroll only if they are not of child-bearing potential OR are on a protocol-approved birth control method.

Exclusion criteria:

• Subject is in their first episode of schizophrenia.
• Subject has other psychotic disorders or bipolar disorder.
• Subject has schizophrenia symptoms from taking another medicine or drug of abuse, or due to a general medical condition.
• Subject has a recent history of substance dependence/dependence, or tests positive for illicit drug.
• Subject has an unstable medical disorder, or any significant medical disorder including autistic disorder, organic brain disease, liver dysfunction, epilepsy or seizures, or is at increased risk of developing cerebrovascular problems like stroke.
• Subject has any significant abnormalities in any of the screening tests (ECGs, labs, physical examinations, etc.).
• Subject poses a current serious suicidal or homicidal risk.
• Subject has a positive pregnancy test, or is lactating or planning to become pregnant within one month of the study.
• Subject has recently or is currently participating in another clinical study.
• Subject is stabilized on their current schizophrenia treatment.
• Subject needs to take any of the medicines not permitted in the study, or has recently had ECT (electroconvulsive therapy) or TMS (transcranial magnetic stimulation).
• Subject was non-responsive to two or more adequate trials of antipsychotic treatments over the past 2 years.
• Subject has had an allergic or significant reaction to any of the study drugs, or can't take risperidone.