The basic morphological properties of liver cells are defined in the form of a morphometric model to permit integrated quantitative characterization of functionally important parameters. Stereologic methods which allow efficient and reliable quantitative evaluation of sectioned liver tissue are presented. Material, obtained by a rigorous three-stage sampling procedure from five normal rat livers, is systematically subjected to this analysis at four levels of magnification. This yields quantitative data which are expressed as "densities," i.e. content per 1 ml of tissue, as "specific dimensions" related to 100 g body weight, and as absolute dimensions per average "mononuclear" hepatocyte. Base line data relating to the normal rat liver are presented for the entire spectrum of parameters. As examples, 1 ml of liver tissue contains 169 x 10⁶ hepatocyte nuclei, some 90 x 10⁶ nuclei of other cells, and 280 x 10⁹ mitochondria. Hepatocyte cytoplasm accounts for 77% of liver volume, and the mitochondria for 18%. The surface area of endoplasmic reticulum membranes in 1 ml of liver tissue measures 11 m² of which are ⅔ of the rough form carrying some 2 x 10¹³ ribosomes. The surface area of mitochondrial cristae in the unit volume is estimated at 6 m². The validity and applicability of the method are discussed, and the data are compared with available information from other studies.