After an overnight fast, blood samples were obtained from seven obese women (50% +/- 3% body fat) and from seven control women (25% +/- 1% body fat) before, during, and after 10 minutes of treadmill exercise at 70% of each individual's maximal oxygen uptake (VO2max). During exercise, peak plasma epinephrine (E), norepinephrine (NE), and glucagon concentrations in the control group significantly exceeded corresponding peak values in the obese group by 1.4-fold to twofold, whereas lactate responses did not differ. After 5 minutes of rest, peak plasma glucose, free fatty acid (FFA), and growth hormone (GH) concentrations in the control group also were significantly higher than in the obese women, but the plasma cortisol responses were comparable. Although plasma insulin concentrations decreased during exercise and rose to maximum values at 5-minute recovery in all individuals, levels were more than 3.5-fold higher in the obese group throughout the study. We conclude that the combination of heightened plasma insulin and diminished catecholamine and other counterregulatory hormone responses may account for subnormal plasma substrate increments that distinguish obese from non-obese women during exercise at comparable work intensities.