The effects of the supplementation of aspartic acid and asparagine (45 mg.kg1 body weight of each), and carnitine (90 mg.kg-1 body weight) during one week on the ultrastructure of soleus muscle from swimming-trained (five weeks) and sedentary rats were examined. In trained rats, the amino acids supplementation was performed during the last week of the exercise training only. Supplementation of these amino acids in the diet either in sedentary and trained rats caused myofibrillar and mitochondrial disorganization and dissolution. Focal degeneration of myofibrils and Z-line streaming and disruption, as well as internalization of nuclei were observed. The size of mitochondria increased and some of them presented severe swelling, with decreased electron-density of the matrix and disruption of internal and external membranes. The changes in the soleus muscle ultrastructure described do suggest functional disorders. This observation is particularly important for the amino acid intakers and deserves to be further investigated.