This open label, non-randomized, parallel group study will evaluate [11C]-RO5011232 as radiotracer for brain mGlu5 receptor occupancy and investigate the binding of RO49917523 to human mGlu5 brain receptor at steady-state in healthy volunteers. In Parts I, II and III, positron emission tomography (PET) assessments of specific uptake of the radiotracer and whole body dosimetry will be made after intravenous injection of [11C]-RO5011232. In Part IV, subjects will receive RO4917523 orally daily for 14 days while receiving three single intravenous bolus injections of [11C]-RO5011232. The anticipated time on study treatment is up to 4 weeks.

Inclusion Criteria:

• Healthy male and female volunteers as determined by medical history, physical examination, 12-lead ECG, and routine clinical laboratory measurements
• Age 18 to 65 years inclusive
• Body mass index (BMI) between 18 to30 kg/m2 inclusive and body weight at least 50 kg
• Appropriate body size (less than 195 cm [6 feet and 5 inches]) in order to accommodate the whole body scanning

Exclusion Criteria:

• History of or presence of clinically significant psychiatric condition, as evaluated by a psychiatrist before enrolment
• History of head trauma with prolonged loss of consciousness (>10 minutes) or any neurological condition or a history of migraine headaches
• Contraindications to MRI procedures (Part I, III and IV)
• Subjects suffering from claustrophobia or who would be unable to undergo magnetic resonance imaging (MRI) or PET scanning
• Positive pregnancy test at screening or on any day preceding a PET scan
• Lactating women
• Positive for hepatitis B, hepatitis C or HIV infection
• Any history of alcohol or drug abuse within the last 6 months
• Regular smoker or nicotine user (>10 cigarettes per day)
• Participation in an investigational drug or device study within three months prior to screening