Glucose is the principal energy substrate for the brain and studies have shown that the brain is able to increase glucose availability in the face of glucose starvation (neuroglycopaenia). The mechanisms, believed to be hypothalamic, that may be involved in a brain/blood glucose control system have not yet been identified. We have used novel techniques for assessing brain monoamine neuronal activity to investigate its relationship to blood glucose concentrations in the rat. We describe here two important relationships which emerge from these studies. One is that activation of hypothalamic noradrenaline (NA) activity following stress is associated with concurrent increases in plasma glucose concentrations. This relationship is linear and independent of the adrenal or pituitary glands. The second is an inverse relationship between plasma glucose concentration and hypothalamic NA neuronal activity--high blood glucose levels significantly inhibited the hypothalamic NA activity responses to stress, alpha 2-adrenergic blockade and adrenalectomy. Thus glucose (or a metabolite of it) seems to provide a negative feedback signal sensed by hypothalamic NA neuronal systems which, in turn, appear to stimulate liver glucose output by a neural mechanism.