The time of day influences the magnitude of adrenocorticotropic hormone (ACTH) and corticosterone responses to hypoglycemia. However, little is known about the mechanisms that impart these time-of-day differences on neuroendocrine CRH neurons in the hypothalamic paraventricular nucleus (PVH). Rats received 0–3U/kg of insulin (or 0.9% saline) to achieve a range of glucose nadir concentrations. Brains were processed to identify phosphorylated extracellular regulated 1/2 kinase (phospho-ERK1/2)-immunoreactive cells in the PVH and hindbrain and corticotropin-releasing hormone (CRH) heteronuclear (hn) RNA in the PVH. Hypoglycemia did not stimulate ACTH and corticosterone responses in animals unless a glucose concentration of approximately 3.15mM or below was reached. Critically, the glycemic thresholds required to stimulate ACTH and corticosterone release in the morning and night were indistinguishable. Yet, glucose concentrations below the estimated glycemic threshold correlated with a greater increase in corticosterone, ACTH, and phospho-ERK1/2-immunoreactive neurons in the PVH at night compared to morning. In these same animals, the number of phospho-ERK1/2-immunoreactive neurons in the medial part of the nucleus of the solitary tract was unchanged at both times of day. These data collectively support a model whereby changes in forebrain mechanisms alter the sensitivity of neuroendocrine CRH to the hypoglycemia-related information conveyed by ascending catecholaminergic afferents. Circadian clock driven processes together with glucose sensing elements in the forebrain would seem to be strong contenders for mediating these effects.