In a previous study we estimated fetal mouse thoracic volume by use of paraffin casts. While this procedure provided useful information, it did not allow histologic examination of thoracic viscera. In the present study the thoracic volumes of day 14–18 fetal mice were determined through serial histological sections. The thoracic cavity was traced from the sections and the area of each tracing was determined by computer image analysis. These areas were summed and then multiplied by the thickness of each section to derive the thoracic volume. This procedure thus permitted both volumetric determinations and histological inspection of the thoracic viscera. In addition, two randomized sampling methods designed to increase the utility of such volumetric estimates were compared for reliability. The method best suited for this study was a random stratified sampling method because it reproduced estimates with minimal standard deviation.