The effects of anabolic steroid treatment in association with endurance training on biochemical serum parameters and liver ultrastructure have been investigated in male rats. Values of serum alanine aminotransferase, aspartate aminotransferase and alkaline phosphatase were not significantly affected by administration of high doses of fluoxymesterone or methylandrostanolone. Electron microscopic examination of hepatic tissue from treated animals revealed ultrastructural alterations of hepatocytes. The most prominent changes were swelling of mitochondria, which presented electron-lucent matrix and slightly defined cristae, and a marked increase in the number of lysosomes. These changes were evident in both sedentary and trained treated rats, indicating that liver cell damage is produced by anabolic-androgenic steroids despite the simultaneous realization of physical exercise. The alterations observed were not detected by means of conventional biochemical liver tests.