In the present study, both of the major enzymes, monoamine oxidase and catechol-o-methyltransferase, involved in the degradation of norepinephrine were examined indirectly in segments of rabbit aorta by exposure of tissues to 14C-norepinephrine and subsequent isolation of metabolites from homogenates by chromatographic techniques. Segments of aorta were preincubated with vasodilators to examine their influence on accumulation and metabolism of 14C-noreρinephrine. The accumulation of labeled norepinephrine by aortae was enhanced after pre-incubation with hydrochlorothiazide or diazoxide, and reduced after preincubation with sodium nitroprusside or nitroglycerin. However, the fraction of accumulated 14C-norepinephrine which was degraded was significantly reduced after exposure to all four vasodilators. These results suggest that a decrease in degradation of norepinephrine in vascular smooth muscle can be observed (1) in vitro and (2) fairly rapidly after exposure to vasodilators