The purpose of this study is to evaluate the safety and efficacy of a 28-day regimen of exenatide (AC2993), given as a monotherapy to subjects with type 2 diabetes mellitus.

Inclusion Criteria:

• Subject has type 2 diabetes mellitus treated with diet and exercise modification alone or in combination with one oral antidiabetic agent for no longer than 4 years.

Exclusion Criteria:

• Subject has a clinically significant history or presence of any of the following conditions: (a) Hepatic disease, (b) Renal disease, (c) Central nervous system disease, (d) Gastrointestinal disease (e) Pulmonary disease (f) Hematologic disease.
• Subject is currently treated with any of the following excluded medications: (a) Metformin/sulfonylurea combination therapy (b) Thiazolidinediones (c) Insulin as outpatient therapy (d) Regular use of drugs that directly affect gastrointestinal motility (e) Regular use of systemic corticosteroids by oral, intravenous (IV), or intramuscular (IM) route, or potent, inhaled, intrapulmonary, or intranasal steroids known to have a high rate of systemic absorption (f) Regular use of medications with addictive potential such as opiates, narcotics and tranquilizers (g) Antineoplastic agents (h) Transplantation medications (i) Prescription weight-loss medications.