The purpose of this study was the evaluation of changes induced by training (swimming 1 h/day, 5 days/week, 6 weeks) and an anabolic hormone (nandrolone decanoate, intramuscular injections of 15 mg.kg-1 per week) on fiber size, capillarization and mitochondrial fraction of murine soleus muscle. The animals (n = 32) were divided into 4 groups: a control group (C), that received the arachis oil carrier; a steroid group (S), that received the hormone; a training group (T), and a group that was submitted to training and to the administration of hormone (S + T). The soleus muscle was selected for quantitative light- and electron-microscopic evaluation. The muscle fiber size was increased in group T and decreased in groups S and S + T. The axial length of capillaries per unit volume of muscle decreased significantly in groups S and T. The number of capillaries per number of fibers showed a significant decrease in groups S and S + T and an increase in group T. The mitochondrial content decreased in group S, which suggested that anabolic steroids can be harmful for these organelles. This hypothesis was confirmed by histological evaluation at the electron-microscopic level. Many swollen and disrupted mitochondria were found in groups S and S + T. The results suggest that administration of nandrolone decanoate may have some deleterious effects on the muscle respiratory system (capillaries and mitochondria).