This 5 arm study will evaluate the efficacy and safety of RO5093151 and RO5027838 in patients with type 2 diabetes mellitus on a stable dose of metformin. After a 4 week pre-randomization period for glucose control, patients will be randomized to one of 5 groups to receive a)RO5093151 400mg po bid b)RO5093151 10mg po bid c)RO5027838 200mg po qd d)RO5027838 50mg po qd or e)placebo po bid for 4 weeks. The anticipated time on study treatment is < 3 months, and the target sample size is 100-500 individuals.