The present paper reviews some recent experiments with relevance to the functional significance of brain noradrenergic systems with bearing on three clinically relevant topics: 1. The withdrawal and abstinence reactions after the antihypertensive agent clonidine as well as after opiates. 2. The mental reactions associated with alterations in blood volume and acid‐base balance. 3. The central actions and side‐effects of β‐adrenoceptor blocking agents. The available evidence indicates that central NA neurons may serve a function within the CNS analogous to that of the peripheral sympathetic nerves, i.e. to alert and alarm the individual to significant events in the external and internal environment. Thus, the largest brain NA system, which emanates from locus coeruleus and innervates vast regions of the neuroaxis, is largely activated in the same situations and by the same mechanisms (e.g. blood‐volume and chemoreceptors) as the sympathetic system, and may provide part of the central machinery for the anxiety reaction associated with hypercapnia as well as the withdrawal reactions after clonidine or morphine. Some, but not all centrally active β‐adrenoreceptor blocking agents were found to affect the activity of brain NA systems. This result suggests that there may be significant differences with respect to the clinical, central side effects of