A Study of The Abuse Liability Potential of RO4917838 in Recreational Drug Users

Inclusion Criteria: * Healthy male or female volunteers, 18 to 55 years of age inclusive * Recreational drug abuse experience (\>/= 10 times lifetime abuse of a CNS depressant, \>/= 1 abuse of CNS depressant in the previous 3 months * Body mass index (BMI) 18 to 30 kg/m2 inclusive * Main study (Part 2): Able to differentiate between diazepam and placebo Exclusion Criteria: * History of any significant disease or disorder * History or current diagnosis of substance dependence (excluding caffeine and nicotine) * Currently seeking or history of participating in treatment for substance-related disorders, including successful completion of such treatment * Any confirmed significant allergic reactions against any drug, or multiple allergies in the judgement of the investigator * Positive for hepatitis B, hepatitis C or HIV infection * Pregnant or lactating women * Participation in an investigational drug or device study within the last 30 days prior to Day 1 of the study * Confirmed positive drug screening at screening / any Day -1 (allowed positive THC at screening and any Day -1 and positive benzodiazepine at screening only) * Positive alcohol breath test at screening / any Day -1 * Heavy smoker (\> 20 cigarettes, \> 8 pipefuls or \> 8 cigars per day)