Dopamine-beta-hydroxylase (DBH), the enzyme responsible for the biosynthesis of noradrenalin from dopamine, was assayed in the blood plasma of 20 men with primary hypertension. At the same time plasma was taken for measurement of plasma renin activity. Renin release as well as the plasma level of DBH is dependent upon the activity of the sympathetic nervous system, at least to some extent. A possible relationship between the two enzymes was therefore investigated. However, no relationship could be found in this series of 20 hypertensive patients. Another aim was to study the levels of DBH in venous and arterial blood simultaneously. No difference in the DBH level was found in venous and in arterial blood in 11 patients undergoing heart catheterization.