Menopause is associated with increased neurokinin B (NKB) gene expression and decreased proopiomelanocortin (POMC) gene expres- sion in the human hypothalamus. In the present study, young, ovari- ectomized cynomolgus monkeys were used in a model of menopause to examine the effects of hormone replacement therapy (HRT) on hypothalamic neuropeptide gene expression. A secondary goal was to determine whether HRT produces signs of estrogen toxicity in the primate hypothalamus by examining POMC neurons and microglial cells. In situ hybridization was performed using synthetic, radiola- beled, 48-base oligonucleotide probes. a-napthyl butyrate esterase histochemistry was used to visualize microglial cells. Both estrogen and estrogen plus progesterone treatments produced a marked sup- pression of the number of infundibular neurons expressing NKB gene transcripts. In contrast, HRT had no effect on the POMC system of neurons or the number of microglial cells in the infundibular nucleus. These results provide strong support for the hypothesis that the increased NKB gene expression in the hypothalamus of postmeno- pausal women is secondary to estrogen withdrawal. Conversely, these data suggest that the dramatic decline in the numbers of neurons expressing POMC gene transcripts in older women is caused by fac- tors other than ovarian failure. Finally, we found no evidence that HRT, in doses designed to mimic currently prescribed regimens, pro- duces signs of estrogen toxicity in the primate infundibular nucleus. (J Clin Endocrinol Metab 84: 2111-2118, 1999)