The existence of dopamine βhydroxylase (DBH)(3,4 dihydroxyphenylethylamine, ascorbate oxygen oxidoreductase, EC 184.108.40.206) was first demonstrated in mammalian tissue by Blaschko in 1957,' although its presence had been predicted 18 years earlier.' Since then, DBH has become one of the most intensively studied protein molecules, not because of any uniqueness in its catalytic or physical characteristics, but mainly because it has provided a convenient marker for the study of noradrenergic nerve and chromaffin cell function. Thus, DBH has been used to localize noradrenergic nerves by both biochemical and immunohistochemical methods. It has been measured in blood as a potential integrative index of sympathetic nerve activity. It has been used as a marker for catecholamine storage vesicles in the investigation of exocytotic mechanisms and as a tracer of axonal transport of these vesicles. It has provided a useful model for the study of the interconversion of active and inactive enzyme, and most recently, the use of DBH antibodies injected in vivo has led to the production of specific immune lesions of both peripheral and centraI nervous tissue. Thus, a review of DBH would not provide an adequate insight into the applications of our present knowIedge of this enzyme if i t were only to describe the physical, chemical, and catalytic properties of the molecule. We have, therefore, chosen to provide a brief description of the basic properties of the enzyme itself and then to highlight, where appropriate, those areas of research that have been influenced by the study of this molecule.