This purpose of this study is to compare the effect of exenatide to that of sitagliptin on 2-hour postprandial glucose in subjects with type 2 diabetes mellitus.

Inclusion Criteria:

• Treatment with metformin for at least 2 months

• Has HbA1c 7.0% to 11.0%, inclusive, at screening

• Body mass index (BMI)25 kg/m^2 to 45 kg/m^2, inclusive

• List of medications that are not allowed or the patient has been on stable treatment for at least 2 months:

  • Hormone replacement therapy (female subjects)
  • Oral contraceptives (female subjects)
  • Antihypertensive agents
  • Lipid-lowering agents
  • Thyroid replacement therapy

Exclusion Criteria

• Has been treated with exenatide (BYETTA®) or any DPP-4 inhibitor prior to screening

• Received any study medication or participated in any type of clinical trial within 30 days prior to screening

• Has donated blood within 60 days of screening visit or is planning to donate blood during the study

• Treated with any of the following medications:

  • Sulfonylurea or Thiazolidinedione within 3 months of screening;
  • Alpha-glucosidase inhibitor, meglitinide, nateglinide, or pramlintide (SYMLIN®)within 30 days of screening;
  • Insulin within 2 weeks prior to screening or insulin for longer than 1 week within 6 months of screening;
  • Drugs that directly affect gastrointestinal motility, including but not limited to, anticholinergics, macrolide antibiotics, dopamine antagonists, opioids, and Reglan®(metoclopramide);
  • Systemic corticosteroids by oral, intravenous, or intramuscular route; or potent, inhaled, or intrapulmonary (including ADVAIR®) steroids known to have a high rate of systemic absorption;