Qualitative and quantitative changes in the liver tissue composition have been studied during prenatal development of the Wistar rat by electron microscopy and stereologic methods. The absolute volume of the fetal liver is multiplied by 84 between days 13 and 20 of gestation. In the meantime, the average hepatocyte volume is multiplied by 1.5 between days 12 and 20. The volumetric fraction of hepatocytes increases from 35% of the volumetric fraction of the liver on day 12 to 66% on day 20 of gestation. The non-hepatocyte cells decrease from 49% on day 12 to 25% on day 20. By days 12 and 13, the rough endoplasmic reticulum and the Golgi apparatus are well differentiated, indicating that young fetal hepatocytes are able to synthesize and export plasma proteins. The volumetric fraction of free ribosomes decreases from 38% of the hepatocytic cytoplasm on days 12 and 13 to 6% on day 20. The mitochondrial compartment occupies about 10% of the hepatocyte cytoplasm. The mitochondria, small and round on days 12, 13 and 14, become oblong from day 18 of gestation. The shape of hepatocytes changes during the prenatal development, from potato-like on days 12, 13, 14 to cubic on day 20, with an intermediate, more spheric, stage on day 18.