This randomized, double-blind, parallel-arm study will assess the safety and tolerability of RO4995819 in combination with citalopram in healthy volunteers. Subjects will receive citalopram orally on Days 1-9 and be randomized to receive either RO4995819 orally or placebo in combination with citalopram on Days 10-16. Safety follow-up will be up to Day 58.

Inclusion Criteria:

• Adult healthy male or female subjects, 18 to 65 years of age inclusive (healthy status defined as absence of evidence of any active or chronic disease)
• Body mass index (BMI) 18.0 to 30.0 kg/m2 inclusive
• Female subjects who are not either surgically sterile or post-menopausal must commit to using a barrier form of contraception in addition to either an intrauterine device or hormonal contraception until at least 5 months after the last dose of study drug
• Male subjects must use a barrier method of contraception throughout the study and for up to 5 months after the last dose of study drug (applies also to surgically sterilized males)
• Willing not to participate in any other clinical trial with an investigational drug for at least 5 months following the last dose of study drug

Exclusion Criteria:

• Pregnant or lactating females
• Suspicion of regular consumption of drug of abuse or positive test for alcohol on Day -2
• Hepatitis B, hepatitis C or HIV infection
• Smokers of >5 cigarettes or equivalent tobacco intake per day
• Any confirmed allergic reaction against any drug, or multiple allergies (non-active hay fever is acceptable)
• Participation in an investigational drug or device study within 3 months prior to dosing