This study is being conducted in 2 groups of schizophrenic patients: in group 1 the relationship between the dopaminergic D2 receptor occupancy (D2 RO ) and serotoninergic 5HT2A receptor occupancy at stable plasma concentration (Cp) of SB-773812 will be investigated using SPECT scan in up to 15 chronic schizophrenic patients in an open- label design. In group 2 up to 80 patients will receive SB-773812 or Risperidone for 6 weeks to obtain at least 10 patients showing clinical improvement with SB-773812. The relationship of D2 RO, measured at steady state after chronic dosing at Cmax (6+/-2 h) and Ctrough (24+/-4 h), and PK at steady state after repeated doses, to efficacy readouts will be investigated. The SB-773812 dose administered will be selected as the highest dose proven to be safe and well tolerated from a previous study.

Inclusion criteria:

• schizophrenic patients as diagnosed by DSM IV criteria.
• In group 2, subject has schizophrenia or schizophreniform disorder and show moderate disease exacerbation, a PANSS score > 65 and a score in the PANSS positive scale (PANSS-P) =25.
• Women of childbearing potential must agree to acceptable method of birth control.

Exclusion criteria:

• Subject shows severe exacerbation, extreme instability, high risk for suicide or serious violent behaviour.
• Any clinically or laboratory significant abnormality.
• Subjects receiving a radiation dose from other activities of more than 10 mSv over any 3 year period.
• Subjects with organic brain disease and history of severe head trauma.
• Heart pacemaker, metallic prosthesis or other metallic body implants.
• Significant head deformity.
• Smokers with associated COPD.
• History or presence of clinically significant gastro-intestinal, hepatic or renal disease.
• History of cholecystectomy or biliary tract disease.
• Positive for HBV, HCV or HIV.
• Pregnant or lactating women.