Dopamine-beta-hydroxylase (DBH) and norepinephrine are both localized in noradrenergic storage vesicles. When noradrenergic nerves fire, both norepinephrine and DBH are released by exocytosis. DBH released from the peripheral nervous system and the adrenal medullae is found in blood, while DBH in cerebrospinal fluid (CSF) is presumably of central origin. This study was designed to: (1) investigate the effect of drugs which alter central noradrenergic activity on DBH activity in CSF; and (2) compare the effects of these drugs on DBH in CSF and plasma in cats. Phenoxybenzamine was given subcutaneously at 6 mg/kg and DBH was measured 8 h later. This treatment significantly increased DBH activity in CSF (n = 10,P less than 0.005). There were no consistent changes in plasma DBH, although there was a tendency for DBH to increase from low basal levels and to decrease from high basal levels. Clonidine was administered in 4 subcutaneous injections (100, 50, 50, 50 microgram/kg) over a 19-h period, and blood and CSF were taken 5 h after the last injection. This treatment caused a significant decrease in CSF DBH activity (P less than 0.05, n = 8). The effect of clonidine on plasma DBH was strongly dependent on the basal enzyme level. The 3 lowest DBH values increased and the 5 highest DBH values decreased on drug treatment. These results are discussed with respect to the theory that changes in CSF DBH may reflect central noradrenergic activity.