The effects of catechol O-methyltransferase (COMT) and monoamine oxidase (MAO) inhibitors on the accumulation and metabolism of [l-3H] norepinephrine ([l-3H]NE) were studied in the isolated adventitia and media of rabbit aorta. The COMT inhibitors produced a greater inhibition of O-methylation in the isolated media than in the isolated adventitia. In addition, exposure to these drugs was associated with an increased level of [l-3H]NE and increased formation of the catechol deaminated metabolites by the isolated media but not by the isolated adventitia. The effects of these drugs on the balance of the oxidized vs. reduced forms of the deaminated metabolites was interpreted to mean that 3-methoxy-4-hydroxyphenylethylglycol is formed from normetanephrine rather than from 3,4-dihydroxyphenylethylglycol whereas 3-methoxy-4-hydroxymandelic acid is formed from 3,4-dihydroxymandelic acid. 3,4-Dihydroxyphenylethylglycol does not seem to serve as a substrate for 3-methoxy-4-hydroxyphenylethylglycol becuase it rapidly passes out of the tissue after it is formed. Parglyline and iproniazid markedly decreased deamination of [l-3H]NE and increased the tissue level of [l-3H]NE. However, these drugs produced little if any compensatory increase in O-methylation of [l-3H]NE. The effects of these drugs are compatible with the hypothesis that adrenergic nerves contain MAO and vascular smooth muscle contains MAO and COMT. Harmaline seemed to block both MAO and COMT. Based on the effects of clorgyline and deprenyl and the other MAO inhibitors tested, the isolated adventitia seems to contain primarily type A MAO and the isolated media seems to contain primarily type B MAO.