The primary purpose of this study is to evaluate the safety and dose-response relationship of N-methylnaltrexone bromide (MOA-728) by observing spontaneous bowel movements in subjects with chronic pain, which is not due to malignant cancer, and who have opioid-induced bowel dysfunction (OIBD).

Inclusion Criteria:

• Adult outpatients with opioid-induced bowel dysfunction and chronic pain, which is not due to malignant cancer.
• Taking oral, transdermal, intravenous, or subcutaneous opioids.
• Willingness to discontinue all pre-study laxative therapy and use only study permitted rescue laxatives.

Exclusion Criteria:

• History of chronic constipation before the initiation of opioid therapy.
• Other GI disorders known to affect bowel transit.
• Women who are pregnant, breast-feeding, or plan to become pregnant.