Catechol-O-methyltransferase (COMT) activity was measured in the aorta and mesenteric arteries of hypertensive rats. In both spontaneously hypertensive rats (SHR) and uninephrectomized-DOCA salt treated hypertensive rats COMT activity per milligram vascular protein increased. The elevated COMT specific activity may be compensatory to the increased blood pressure and serve to increase catecholamine degradation. In contrast to the degradative enzyme, the activity per milligram vascular protein of a synthetic enzyme, dopamine- β-hydroxylase (DBH) decreased by 50% in the mesenteric artery of the SHR. We suggest the possibility that in some types of hypertension both the enzymes of synthesis and degradation may be altered simultaneously to regulate neurotransmitter activity.