Materials and Methods.-Animals and tissues: The major part of the work was carried out with albino Swiss mice bred at the National Institutes of Health. Livers were also obtained from normal and obese (ob/ob) C57 BL/6J mice, Sprague-Dawleyrats, and Fisher rats bearing trans-plantable hormone-secreting sarcomas. Humanliver was obtained at autopsywithin 6 hr of death. In no case was the liver tissue affected by the pathologic process that caused death. Preparation of parenchymal liver cells: Livers from all sources were separated into isolated single cells by the method of Rappaport and Howze. 2 This method has the advantage that prior perfusion of theorgan, as with many of the techniques employing citrate, is not required. The livers were minced with razor blades and then stirred for 1 hr at 250 in a" dissociating" solution of 3 X 10-3 Msodium tetraphenylboron (TPB) in a 0.005 M sodium phosphate buffer, pH 7.8, containing 0.05 M sucrose and 0.14 M NaCl. The cell suspensions were then decanted and washed twice with cold dissociating solution without TPB. For spectrophotometric work, the washed cells were suspended in the dissociating solution with-out TPB, and 1 mlof cell suspension (15,000/ml) was collected with gentle suction on a 19 X 42-mm Millipore filter with a poresize of 5.0 A (SMWPO19OR). The filters were fixed in absolute ethanol, clipped onto 1 X 3-in. glass slides air-dried, and stained for DNA by the Feulgen method with a counterstain of Fast Green FCF. 3 After staining, the filters were dehydrated in ethanol, cleared in xylene, and mounted in Permount (Fisher Scientific Co.). Quantitative microspectrophotometry of the Feulgen-stained nuclei was carried out at 540 mj with a Barr and Stroud GN2 integrating microspectrophotometer. The readings for individual nuclei clustered tightly about modal values that were in the ratio of 2: 3.7-3.9: 7.0-7.4. The departure of these ratios from the expected 2: 4: 8 appears to be the results of systematic errors introduced by the measurement of objects of different sizesin an aperture of fixed size. Determination of cell size: Because of the difficulty in accurately measuring thevolumes of single cells, cell size was determined from cell areas. The cells were suspended in the dissociating solution without TPB, and trypan blue at a final concentration of 1: 1000 was added to stain the nuclei. To prevent distortion of their shapes, the cells were placed in a hemocytometer chamber