In peripheral tissues, noradrenergic nerves have a diffuse distribution, but in the brain, noradrenergic neurons arise in the brainstem and are inhomoge-neously distributed. The locus coeruleus is the largest noradrenergic nucleus in rat brain, and it contains only about 1500 neurons. Although norepinephrine is present in the brain in small amounts, it has been extensively studied for many reasons. Central norepinephrine is involved in the regulation of blood pressure and emotions, and the two most common diseases in Western civilization are hypertension and mental illness. Many of the drugs used in the treatment of these and other diseases interact with norepinephrine. Sensitive fluorometric and radioenzymatic assays make analysis of the small amount of norepinephrine present in the brain possible so that we know a great deal about this chemical that occurs in such small amounts.