Purpose and Methods: To determine whether pregnancy modifies the effect of aerobic exercise on insulin responsiveness, female rats were mated or kept nonpregnant and exercised or not on a treadmill (10° slope, 20 m·min-1) 5 d·wk-1 during a 20-min period that was increased progressively up to 70 min on the 19th d. On day 20, a hyperinsulinemic euglycemic clamp was performed with 0.8 IU insulin·h-1·kg-1 under conscious conditions. Results: Food intake and body weight, circulating lactic acid, glucose, and insulin as well as fetal body weight and number were unaffected by the exercise protocol. The rate of glucose infusion required to maintain basal glucose levels during the clamp was similar in exercised and nonexercised virgin rats and significantly lower in pregnant than in virgin nonexercised rats. However, in exercised pregnant rats the glucose infusion rate was almost as high as in the exercised virgin rats. Conclusions: The results show that although our aerobic exercise protocol does not affect insulin responsiveness in nonpregnant rats, it completely reverts the insulin resistance present in late pregnant rats.