Purpose: The clinical implications of metformin attenuating the insulin sensitizing effects of exercise are unclear. We tested the hypothesis that metformin would blunt reductions in post-prandial glucose in an exercise intensity-based manner in adults with metabolic syndrome (MetS) risk. Methods: In a double-blind trial, participants were randomized to low intensity exercise plus placebo (~55%VO 2 max 5d/wk, LoEx+PL, n=22) or metformin (2000mg/d, LoEx+Met, n=21) and high intensity exercise plus placebo (~85%VO 2 max 5d/wk, HiEx+PL, n=24) or metformin (HiEx+Met, n=24) for 16 weeks. A 120-min 75g oral glucose tolerance test was conducted pre- and post-treatment to assess early (0-30min), late (60-120min), and total (0-120min) phase glucose tolerance and circulating insulin (total area under the curve; tAUC). Results: HiEx+Met increased 1-hr and 2-hr glucose by ~18-21mg/dl ( P P =0.008). Late phase and total phase glucose tAUC was also higher after HiEx+Met versus HiEx+PL and LoEx+PL (both, P P , P <0.04) after exercise training only. Conclusions: Metformin increased post-prandial glucose after high, but not low, intensity exercise training in adults with MetS risk.