The aim was to investigate the changes in lipid peroxidation, antioxidant enzyme activities, and muscle damage in the same and different exercise intensities during walking and running. Fourteen healthy males participated in this study. The subjects' individual preferred walk-to-run transition speeds (WRTS) were determined. Each subject covered a 1.5-mile distance for 4 exercise tests; walking (WRTS-W) and running (WRTS-R) tests at WRTS, 2 km·h -1 slower walking than WRTS (WRTS-2) and 2 km·h-1 faster running than WRTS (WRTS+2). Blood samples were taken pre, immediately, and 30 minutes post each test. The changes in (MDA) and glutathione (GSH) levels and superoxide dismutase (SOD), catalase (CAT), and creatine kinase activities were measured. Oxygen uptake, carbon dioxide output, oxygen uptake per kilogram of body weight, and heart rate during exercises were significantly higher in both the WRTS-W and the WRTS+2 exercises compared with the WRTS-2 and WRTS-R. Oxygen consumption and energy expenditure were higher in walking than in the running exercise at the preferred WRTS and only WRTS-W exercise significantly increased MDA levels. Catalase activities were increased by WRTS-W, WRTS-R, and WRTS+2 exercises. Changes in SOD and CAT activities were not different between walking and running exercises at the preferred WRTS. Total plasma GSH increased in response to WRTS-W exercise, which could be associated with an increase in MDA. Also, total GSH levels 30 minutes postexercise were significantly lower than postexercise in WRTS-2, WRTS-W, and WRTS+2 exercises. Our results indicate that walking and running exercises at the preferred WRTS have different oxidative stress and antioxidant responses. © 2010 National Strength and Conditioning Association.