Neuron density, volume of the area and total neuron number were measured in the medial preoptic area (MPOA), supraoptic nucleus (SO)paraventricular nucleus (PVN), anterior hypothalamic area (AHA), ventromedial nucleus (VMN), dorsomedial nucleus (DMN) and arcuate nucleus (ARN) of the hypothalamus in young adult female rats (3.5–5 months old) and in old female rats (over 24 months old). As age advances there is a tendency to increase in the volume of hypothalamic area and consequently decrease in the neuron density. However, loss of neuron due to aging occurs only in MPOA, AHA and ARN. The decrease in neuron number due to aging in these three areas ranged from 23 to 50%. No neuron loss due to aging was observed in SO, PVN, VMN and DMN. The physiological significance of neuron loss in MPOA, AHA and ARN is discusse