In unanaesthetized rats chronically prepared with venous and intracerebral cannulae, noradrenaline injected into the region of the supraoptic nuclei caused a dose-dependent increase in plasma vasopressin, measured by radioimmunoassay. A similar response was obtained with phenylephrine, but not with either clonidine or isoprenaline. The secretion of vasopressin was not secondary to change in arterial pressure, since similar injections of noradrenaline resulted in a small increase in arterial pressure, measured in the anaesthetized rat. These results suggest that noradrenaline stimulates alpha-1-adrenoceptors, presumably located on vasopressin-secreting neurones, thereby causing these cells to secrete vasopressin into the circulation. Tyramine injections also resulted in a prompt elevation in plasma vasopressin, indicating that endogenous noradrenaline is capable of releasing vasopressin.