1. The hepatic reticulo-endothelial cell population is generally assumed to increase in size, along with the liver, during ageing in rats. However, this has not been rigorously established. 2. Using electron microscopy and stereological techniques, the present study has shown that the volume densities of hepatocytes and Kupffer cells (and probably also of endothelial cells, fat storing cells and the extracellular space) of the livers of female Sprague-Dawley rats are the same at 2 and 24-25 months of age. 3. This result indicates that the increase in size of the liver during ageing in the rat is associated with an equivalent increase in the volume of each cell population and the extracellular space.