Objective To determine if there is atrophy of neuronal nuclei in the aging rat hippocampus.  Methods  The entire hippocampal formation in a cerebral hemisphere (on a random side) was removed from normal male SD rats aged 1, 2 and 3 years (each age-group 5 rats), serial sections (section thickness 40 μm) were cut after embedding in a resin of glycol methacrylate, and 3-4 sections were systematically sampled and stained with periodic acid, Schiff’s reagent and hematoxylin. The nuclear sizes of hippocampal neurons were estimated with stereological methods, first, the nuclei were sampled in a uniform random way using the optical disector; then, a Feret diameter of the largest nuclear profile at its focal plane (optical section) was measured as the nuclear diameter, and the nuclear volume was measured at the clearest focal plane of the nucleolus with the nucleator.   Results The mean diameters and volumes of the neuronal nuclei in the whole hippocampus were 8.7-9.2 μm and 392-457 μm3, respectively, in the 3 age-groups, without significant statistical difference between groups. There were no significant difference between groups, either, in the nuclear diameter or volume in each hippocampal region——the CA (hippocampus proper), dentate gyrus or subiculum.   Conclusion  In general, no age-related changes have been observed in the nuclear sizes of neurons in the whole hippocampus in the rat from middle to old age.