The presence of dopamine beta-hydroxylase activity in the cerebrospinal fluid of rabbits has been shown using a sensitive radiochemical assay; the identity of the reaction product was confirmed by using thin layer chromatographic techniques. The enzyme found in this fluid has some properties (sedimentation and electrophoretic migration) in common with the best characterized preparation of dopamine beta-hydroxylase, that prepared from bovine adrenal chromaffin granules. It also has these properties in common with the enzyme present in the high-speed supernatants obtained from osmotically disrupted synaptosomes prepared from rabbit brain. When the sciatic nerves of rabbits under urethane anaesthesia were stimulated, or when shaved rabbits were subjected to cold stress, the level of dopamine beta-hydroxylase in the cerebrospinal fluid increased. The increase in response to nerve stimulation was gradual, starting within 90 min of stimulation and remained high for at least 3 h after the stimulation had ended, at which time it was 280% of the normal value. There was no equivalent increase in the protein concentration of the cerebrospinal fluid nor was there a change in the enzyme activity when sciatic nerves were exposed but not stimulated. The enzyme present in the cerebrospinal fluid during this period of high activity is identical in its sedimentation and electrophoretic properties to that present in normal fluid. It is suggested that the dopamine beta-hydroxylase activity in cerebrospinal fluid may be derived from noradrenergic neurons within the brain and that the enzyme is released together with noradrenaline.