The principles of stereology have been applied to a morphometric analysis of parenchymal cells from the peripheral, midzonal, and central regions of normal rat liver lobules. The fractional volumes of cytoplasm occupied by mitochondria, peroxisomes, lysosomes, lipid, and glycogen have been determined. The surface densities of smooth- and rough-surfaced endoplasmic reticulum and of mitochondrial envelope and cristae have also been measured. The average number and dimensions of mitochondria and peroxisomes have been evaluated. By the use of an independent measurement of the average cytoplasmic volume, these data have been expressed as the actual volumes, areas, and numbers per cell in the different parts of the hepatic lobule. Similarly, the volumes of the envelope, cristae, and matrix compartments and the area of cristae membranes have been calculated for the average-sized mitochondrion in each lobular zone. Structural homogeneity is found in over 80% of normal rat liver parenchymal cells, with most of the significant differences being confined to those cells immediately surrounding the central veins.