Hepatocyte morphology of the three zones of the liver lobule was evaluated by stereological analysis in 3-month-old and 35-month-old virgin female WAG/Rij rats. In the old rats, the volume of the nucleus and cytoplasm, and volume densities of smooth endoplasmic reticulum and lysosomes were increased in all areas of the liver lobule, while the volume density of glycogen was decreased. In addition, mitochondrial and vacuolar densities decreased in midzonal hepatocytes. These results are discussed in comparison with other (conflicting) literature data, and the relevance of a more uniform choice of ages, breeding conditions, sex, strain and other secondary criteria is emphasized. The importance of a subdivision of the liver lobule into zones is stressed, as it allows the consideration of environmental factors in the interpretation of morphological changes with age.