1. Serum dopamine β-hydroxylase activity was determined in normotensive control subjects and patients with labile or established essential hypertension. The enzyme activity was 25·9 ± 1·9 (sem), 29·6±2·5 and 25·1 ± 1·9 μmol min—1 1—1, for control, labile and established hypertensive subjects respectively.

2. Neither blood pressure nor serum dopamine β-hydroxylase activity was changed in normotensive control subjects by administration of phentolamine; however, in patients with essential hypertension blood pressure was significantly decreased (P < 0·01) and serum dopamine β-hydroxylase activity was slightly increased. With propranolol administration, blood pressure and the serum enzyme activity were not significantly changed in normotensive or hypertensive subjects.

3. Our results suggest that there is no correlation between serum dopamine β-hydroxylase activity and blood pressure.

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