The aim of the study was to assess the effects of a combination of anabolic-androgenic steroid abuse and endurance training during adolescence on selected aspects of oxidative stress and antioxidant defenses in various striated muscle types. The effects were studied of testosterone propionate (TP) treatment (8 and 80 mg/kg/week, for 6 weeks), given alone or in combination with moderate-intensity endurance training, starting at adolescence, on thiobarbituric acid-reactive substances and heat shock protein 72 (Hsp72) contents, and androgen receptorm(AR) mRNA level in the heart left ventricle, soleus and extensor digitorum longus of male Wistar rats. TP treatment alone markedly elevated thiobarbituric acid-reactive substances only in the left ventricle and soleus; this effect was but marginally enhanced by endurance training. The training alone markedly elevated Hsp72 content in all muscles studied. TP treatment alone dose-dependently upregulated Hsp72, while the lower TP dose slightly curtailed the effect of the training. Low-dose TP treatment alone elevated, whereas high-dose TP treatment alone lowered androgen receptor mRNA level in the soleus and extensor digitorum longus. Endurance training alone elevated AR mRNA in all muscles studied, whereas TP treatment dose-dependently counteracted this effect. Exercise-associated rise in body temperature was significantly less in the TP-treated rats. We came to the conclusion that chronic suprapharmacological TP treatment might exert a protective effect on muscle cell proteins in adolescent sedentary rats, but it markedly enhanced lipid peroxidation. These effects were unlikely to result from an androgen receptor-mediated genomic action of testosterone. Exercise-related heat stress, and not oxidative stress, was mainly responsible for Hsp72 upregulation in striated muscles of chronic TP-treated endurance-trained adolescent male rats.