Previous work has shown that high-intensity (HI) exercise results in total body increased production of lipid peroxidation. These results suggest that exercise induces a higher level of oxidative stress in muscles leading to the production of various peroxides and aldehydes that are potentially toxic to cells. However, these past studies were carried out only with subjects that were exercised to exhaustion or at a very high intensity. In this paper, we report how a moderate-intensity (MI) exercise, which is more normally experienced, as well as a HI acute bout of exercise influenced oxidative stress-related reactions by measuring malonaldehyde (MDA) and lipid hydroperoxides (LH) in red vastus, white vastus, and soleus muscle. The muscles from untrained male Sprague-Dawley rats were removed immediately after either a HI 1-min run at 45 m/min (n = 8) or a 20-min MI run at 20 m/min (n = 8) and compared with a control group that did not run. MI exercise resulted in a 90% increase in MDA in white vastus and a 62% increase in red vastus muscle (P less than 0.05). HI exercise resulted in a 157, 167, and 83% increase in MDA in white vastus, red vastus, and soleus muscle, respectively. LH values in red and white vastus after HI exercise increased an average of 33%, but this proved not to be statistically significant. These results confirm earlier studies that HI exercise does increase MDA in skeletal muscle, and when intensity of exercise is considerably lowered, elevated MDA is still found but at a relatively lesser amount.