Intracerebroventricular (ICV) administration of the hypothalamic neuropeptide neuromedin U (NMU) or the adipostat hormone leptin increases plasma ACTH and corticosterone. The relationship between leptin and NMU in the regulation of the hypothalamo-pituitary adrenal (HPA) axis is currently unknown. In this study, leptin (1 nM) significantly increased the release of CRH from ex vivo hypothalamic explants by 207 ± 8.4% (P < 0.05 vs. basal), an effect blocked by the administration of anti-NMU immunoglobulin (IgG). ICV administration of leptin (10 µg, 0.625nmol) increased plasma ACTH and corticosterone 20 min post injection [(plasma ACTH (pg/ml): vehicle 63 ± 20; leptin, 135 ± 36; P < 0.05) (plasma corticosterone (ng/ml): vehicle, 285 ± 39, leptin, 452 ± 44; P < 0.01)]. These effects were partially attenuated by the prior administration of anti-NMU IgG. Peripheral leptin also stimulated ACTH release, an effect attenuated by prior ICV administration of anti-NMU IgG. We examined the diurnal pattern of hypothalamic NMU mRNA expression and peptide content, plasma leptin and plasma corticosterone. The diurnal changes in hypothalamic NMU mRNA expression were positively correlated with hypothalamic NMU peptide content, plasma corticosterone and plasma leptin. ICV administration of anti-NMU IgG significantly attenuated the dark phase rise in corticosterone [corticosterone (ng/ml), vehicle, 493 ± 38; NMU IgG, 342 ± 47 (P < 0.05)]. These studies suggest that NMU may play a role in the regulation of the HPA axis and partially mediate leptin-induced HPA stimulation.