This is a randomized (assigned study drug by chance), double-blind (both the patient and investigator do not know whether patient is assigned to receive study drug or placebo), placebo-controlled study involving patients with Type 2 Diabetes Mellitus who are taking metformin. Patients are randomized to one of 4 treatment groups and receive study drug (R256918: 5 mg, 10 mg, or 15 mg twice daily) or placebo for a period of 12 weeks. The study consists of a screening period, a baseline visit, the treatment period of 12 weeks, and a follow-up visit. During the treatment period, patients visit the center every two weeks. On each visit, patients come to the clinic after 8 hours fasting. Effectiveness assessments from a blood sample include levels of fasting hemoglobin type A1c (HbA1c), glucose, cholesterol, triglycerides, insulin, C-peptide, and gastrointestinal hormones. Patients monitor their fasting blood glucose and record it in a diary. Insulin sensitivity and beta cell (insulin-producing cells in the pancreas) function are evaluated. Waist and hip circumference are measured. Safety evaluations, including incidence of adverse events, clinical laboratory results, vital signs, and electrocardiograms, are performed during the study. Patients complete a questionnaire related to treatment satisfaction of study medication. Patients receive dietary counseling and will be instructed to remain on a calorically appropriate diet with a maximum of 30 percent (%) of calories derived from fat (in accordance with local practice guidelines for the treatment of type 2 diabetes mellitus), during the entire study. Patients continue taking metformin in the same dose and according to the same dosing regimen as before the study. The overall duration of the study for each patient is approximately 18 weeks. The study will evaluate the effect of R25691 on glucose-dependent insulin secretion, weight loss and changes in insulin sensitivity and tolerability in type 2 diabetic patients.