Nuclear volumes (NV) of neurons in the preoptic area (POA), suprachiasmatic nucleus (SC), paraventricular nucleus (PVN), supraoptic nucleus (SO), anterior hypothalamic area (AHA), arcuate nucleus (ARN), ventromedial nucleus (VMN), dorso-medial nucleus (DMN) and lateral mammillary nucleus (ML) were measured in 3.5- to 5-month-old female and male rats and in old female and male rats over 22 months of age. Young adult female rats had larger NV than males in the neurons of all measured areas except for the SC during estrus and the ARN. NV of the hypothalamic neurons of female rats decreased in old age in all measured areas except for the VMN of rats exhibiting prolonged vaginal cornification (PVC). Old male rats showed decreased NV only in the neurons of the SC, SO and ML. The extent of NV decrease of the hypothalamic neurons was not similar in different areas. The sexual difference in neuronal NV in most of the areas disappeared in old age. Only the POA of rats with PVC and the ML of old female rats had larger NV than those of old male rats.