This is a double-blind, randomized, placebo-controlled, phase I study in normal healthy volunteers. Study treatment includes single doses of MNTX and placebo. In addition, half of each active treatment group is further randomized to a second, replicate dose of MNTX, and the placebo group receives an additional placebo dose.

Inclusion Criteria:

1. Weight between 55 and 85 kg
2. In good health with no evidence of a clinically significant chronic medical condition
3. Non-Smokers.

Exclusion Criteria:

1. History of asthma, allergic skin rash, significant allergy or other immunologic disorder
2. Known or suspected hypersensitivity to opioids or opioid antagonists
3. History or suspicion of alcohol or drug abuse.