The improvement in glucose tolerance is associated with enhanced insulin action in muscle and adipose tissues. Recent reports indicate that hepatic insulin resistance also is improved, but the contribution of increased hepatic insulin action on glucose tolerance is controversial and remains to be thoroughly investigated. Studies indicate that RYGB reduces basal glucose production, and at 1 week following surgery hepatic insulin resistance is improved and insulin clearance increased. In contrast, insulin sensitivity in extra-hepatic tissues is not increased until 3 months after surgery. These observations imply that the liver plays an important role for early improvements in carbohydrate metabolism after RYBG, while increased insulin sensitivity in extra-hepatic tissues (muscle and adipose tissues) is delayed and more closely related to weight loss.