Thus, HFD affects ARC NPY or POMC neuronal cell volume number not cell number. No differences in ARC ionized calcium-binding adapter molecule 1 cell number were observed in any group. ARC glial fibrillary acidic protein cells increased after 2 months on a HFD, although no significant difference after 6 months on chow diet or HFD was observed. HFD reduced orexin cell volume but had a bimodal effect on volume distribution with increased cells at relatively small volumes and decreased cells with relatively large volumes. However, HFD significantly increased average NPY cell volume and affected NPY and POMC cell volume distribution. HFD exposure did not affect total neuronal number in any population. Stereological analysis indicated approximately 8000 NPY and approximately 9000 POMC neurons in the ARC, and approximately 7500 orexin neurons in the lateral hypothalamus. We used design-based stereology to estimate NPY and POMC neuronal number and volume, as well as glial fibrillary acidic protein (astrocyte marker) and ionized calcium-binding adapter molecule 1 (microglia marker) cell number in the ARC as well as orexin neurons in the lateral hypothalamus. However, no studies have examined total neuronal number and volume after high-fat diet (HFD) exposure using sophisticated stereology. The hypothalamic arcuate nucleus (ARC) contains 2 key neural populations, neuropeptide Y (NPY) and proopiomelanocortin (POMC), and, together with orexin neurons in the lateral hypothalamus, plays an integral role in energy homeostasis.