The beta adrenergic agonist isoproterenol has been used in these studies to elevate circulating levels of angiotensin II. Neurons in the brain responsive to the subcutaneous infusion of isoproterenol were identified using an antibody to Fos, the protein product of c-fos which is now used extensively as a marker of activated neurons. Fos-positive neurons were present in a range of specific forebrain and hind brain regions. Infusion of losartan (an angiotensin II type receptor antagonist) showed that neurons in the lamina terminalis were activated directly or indirectly by angiotensin II, whereas other neurons in the hypothalamus and brain stem were responsive as a consequence of the peripheral vasodilation caused by isoproterenol. The distribution of activated neurons in the lamina terminalis was consistent with that of neurons thought to be involved in water drinking.