The objective of this study was to analyze the oxidative stress of skeletal muscle of sedentary rats at the morphological and biochemical level, due to acute physical effort performed at different intensities and during different periods of time. Forty-two male sedentary Wistar rats were divided into two groups, group A (swimming for 50 min) and group B (swimming for 100 min), which were further subdivided into 3 different exercise intensities, non-weight bearing (subgroup I), 3% weight load (subgroup II), and 5% weighted load (subgroup III), as well as a control-rested group (C). The animals were killed by ether inhalation and fragments of the gastrocnemius muscle were removed for the determination of malondialdehyde (MDA), reduced glutathione (GSH), and vitamin E concentrations. When all groups were compared with the control-rested group (C), gastrocnemius MDA levels at 50 and 100 min were higher at all swimming intensities. GSH consumption was greater at all intensities in group A, and only at 100 min in group BI; vitamin E consumption was significantly higher only in groups BII and BIII. When groups were compared by intensity at each swimming time there were no differences between I, II, and III at 50 min for any of the substances analyzed (MDA, GSH, and vitamin E), but at 100 min opposite effects were observed for GSH and vitamin E, with vitamin E consumption and GSH recovery occurring with increasing weight load. Higher concentrations of skeletal muscle MDA could indicate elevated lipid peroxidation at each time and exercise intensity, with highest MDA levels observed after the longest exercise duration and at the highest intensity. The layer-line mode of utilization of body antioxidants could explain the concentrations of GSH and vitamin E: first the use of GSH, even with minor oxidative stress, and then vitamin E consumption only in the presence of greater and more prolonged oxidative stress, indicating the necessity of adequate body levels of both antioxidants for proper utilization by the body antioxidant defense system.