Samples of rat livers were fixed in glutaraldehyde, contrasted en bloc with phosphotungstic acid, embedded in an epoxy resin and serially sectioned. The study of three-dimensional models of 20 complete nuclei shows that all of them share some general features: they have more than one nucleolus (2-4), an irregular layer of compact chromatin adjacent to the nuclear membrane and well-delimited clumps of chromatin both in the nuclear sap and surrounding the nucleoli. A space of 8 sections containing the central nucleolus and a lateral one was studied in detail. In this space, 8 clumps of compact chromatin were found in 17 nuclei and 9 clumps in the other 3 nuclei. No other number of clumps was found in those zones. In all the nuclei studied the compact chromatin surrounding the central nucleolus contacts the nuclear envelope. This contact takes place in a region almost diametrically opposed to the lateral nucleolus in 13 nuclei. In 7 nuclei, these structures were at angles between 50 and 125 degrees. These results support the existence of nonrandom repetitive patterns of chromatin distribution in liver cells.