This is an open-label, Phase I study. Study treatments will entail a four-way crossover among three single, ascending, subcutaneous doses and one intravenous dose of MNTX in six healthy normal male volunteers. Blood samples will be obtained to determine plasma pharmacokinetics, dose proportionality for subcutaneous doses, and absolute bioavailability versus an intravenous dose.

Inclusion Criteria:

1. Subject weight between 60 and 80 kg
2. Subject a non-smoker
3. Subject in good physical health, based on history, physical exam, appropriate laboratory and diagnostics tests at screening with no evidence of clinically significant chronic medical condition.

Exclusion Criteria:

1. Subject with known hypersensitivity to methylnaltrexone, naltrexon or other opioids
2. Subject consuming any prescription medication within the past two weeks, over-the-counter (OTC) products within seven days, any experimental medication within 30 days prior to study screening, or of any drug with a half-life longer then three days within 10 half-lives (>30 days) of screening
3. Subject consuming barbiturates or other inducers or inhibitors of CYP450 within 3 months of screening
4. Subject with history or evidence of cardiovascular, gastrointestinal, hepatic, neurological, pulmonary, and renal or other significant chronic illness.