The boundaries of subnuclei of the hypothalamic paraventricular nucleus (PVN) were outlined on the basis of a step by step computer-assisted image analysis of coronal, serial sections of the hypothalamus in rats. The cell size-frequency histogram, cell packing density, and the cell number were determined for each PVN subdivision. Three principal cell types were distinguished: small, short diameter (d) = 6-10 microns; medium-sized, d = 10-13 microns; and large, d = 13-19 microns. None of these cell types occur exclusively in any PVN subdivision. According to the predominant cell type, the PVN can be divided into magnocellular, mediocellular and parvicellular subnuclei. Large cells appear to form one subdivision in which the cell packing density is relatively stable. Although the cell sizes showed a relatively large variation, no definitive subgroups (parts) could be distinguished. On the basis of cell packing density, the parvicellular and mediocellular subnuclei can be further divided. The mediocellular cell group can be divided into a dorsal and a posterior subdivision. The parvicellular zone is subdivided into a periventricular and a large medial subdivision. Based on inhomogeneities in cell packing density, the medial subdivision can be further divided into anterior, dorso-lateral, ventromedial and caudal parts. The estimated total cell number in the PVN (in both sides of the brain) is about 21,500. On average, 19% of this population, i.e., 4,200 neurons, was calculated in the magnocellular (neurosecretory) division. Out of this, only 66%, about 2,700 cells, falls into the category of large cells. There are approximately 5,700 neurons in the posterior and dorsal mediocellular, and about 11,600 in the periventricular and medial parvicellular subdivisions.