1. Components of the renin-angiotensin system (RAS) are found in the brain; both outside and inside the blood-brain barrier. 2. Almost all of the classical actions of the brain RAS are attributable to angiotensin (Ang) II and mediated by AT1 receptors. 3. Circumventricular organs (CVO), which lack the blood-brain barrier, are rich in AngII receptors and monitor circulating AngII levels. In vivo binding studies suggest that the CVO are also accessible to cerebrospinal fluid-derived AngII. 4. The median preoptic nucleus, paraventricular hypothalamic nucleus, supraoptic nucleus, nucleus tractus solitarius and ventrolateral medulla are inside the blood-brain barrier and are sites of action of brain AngII. In these nuclei, AngII seems to act as an excitatory neurotransmitter or neuromodulator. 5. Actions of AngII in the brain, both inside and outside the blood-brain barrier, are implicated in the central regulation of blood pressure and sympathetic outflow, release of hypothalamic and pituitary hormones and renal sodium handling. 6. Alterations in the activity of brain AngII may be involved in the mechanisms of some types of hypertension.