Intense exercise accompanied by a manifold increase in oxygen utilization over resting conditions has been shown to elevate the probability of the appearance of free radicals. One of the effects of free radicals appears to be the peroxidation of cell membrane lipids resulting in malondialdehyde formation, which is detrimental to cell function. A common method for the measurement of malondialdehyde involves a reaction with thiobarbituric acid. The aim of this study was to evaluate the influence of exercise on concentrations of thiobarbituric acid-reactive substances (TBARS) in the gastrocnemius and soleus muscles of rats. Thirty male rats were used in this study. Soleus and gastrocnemius muscle biopsies were performed and TEARS levels were studied in these two muscles. The rats were randomly assigned to three groups of ten each. Group A was the control group and did not perform exercise. In group B1 gastrocnemius and soleus muscle biopsies were performed immediately after exercise, and in group C muscle biopsies were performed on the 2nd postexercise day. The exercise resulted in a significant increase in TEARS in the gastrocnemius muscle. In the soleus, TEARS also increased, but this was not statistically significant. In conclusion, exercise-induced free radical changes may depend on the muscle type studied.