Catecholamines, namely, dopamine, norepinephrine and epinephrine, play important roles in higher animals as neurotransmitters or hormones, and are metabolized by catechol-O-methyltransferase (COMT). To elucidate the role of COMT in blood pressure regulation, we have developed simultaneous determination methods for catecholamines and their 3-O-methyl metabolites using high-performance liquid chromatography (HPLC)-peroxyoxalate chemiluminescence reaction detection. Using the developed method, we have found that inactivation of catecholamines by COMT is attenuated in hypertensive rats compared to normotensive rats. Furthermore, both the activities and the amounts of membrane-bound (MB-)COMT in the liver were found to be lower in hypertensive rats than in normotensive rats, which indicated that liver MB-COMT may be a relevant factor in blood pressure regulation.