This initial proof-of-concept study focuses on evaluating whether an NK1 antagonist, aprepitant, can block opiate reward in non-dependent opiate experienced volunteers in response to a standard opiate challenge. Sixty subjects will be included in a randomized controlled study. Following a training challenge session, they will receive 1 week treatment with aprepitant or matching placebo, followed by a challenge session during which subjective and physiological responses to the opiate partial agonist buprenorphine will be assessed.