SUMMARY The activity of serum dopamine-hydroxylase (DBH) was measured in 1194 asymptomatic middle-aged men with diastolic blood pressure ranging from 75 to 125 mm Hg during the baseline examination of a multifactorial intervention program for primary prevention of coronary heart disease. No correlation was present between serum DBH activity and systolic (r = − 0.01, NS) or diastolic (r = + 0.02, NS) blood pressure. No significant differences in serum DBH actirity was observed between individuals with blood pressure in the lower, middle or upper deciles. Serum DBH activity was similar in subjects with normal blood pressure, in individuals with widely fluctuating blood pressure and patients with fixed hypertension. The results suggest that serum DBH activity cannot be used as an aid in the diagnosis of essential hypertension of middle-aged men.