The possible contribution of cerebrovascular monoamine oxidase (MAO) to the blood-brain barrier to catecholamines was studied in isolated porcine and rat microvessels by determining its activity with various substrates. Michaelis-Menten kinetic constants, Km and Vmax, were determined using noradrenaline (NA) as substrate in a Tris medium. Km values were 0.25 +/- 0.05 mM in control and 0.16 +/- 0.09 mM in ultrasonically disintegrated (USD) preparations (difference not significant); Vmax in USD preparations (1.83 +/- 0.20 n.atoms O2 min-1 mg protein-1) was slightly higher (p less than 0.05) than in control preparations (1.35 +/- 0.11 n.atoms O2 min-1 mg protein-1), suggesting a certain restriction by the plasma membrane of substrate access to the enzyme. This phenomenon was confirmed in a more physiological, ionic medium; the activity was then approximately doubled for 1 mM NA, whereas that for 1 mM beta-phenylethylamine (beta-PEA), a lipid-soluble substrate, tended to decrease with USD treatment. These results show that this highly active form of MAO is unlikely to be saturated by physiological concentrations of catecholamine. It can be estimated that, for a plasma concentration of NA of 1 microM, a facilitated diffusion accelerating the entry of the catecholamine into the cells by at least 15-fold would be necessary in order to exceed the catabolic capacity of MAO. It is concluded that circulating catecholamines are not likely to cross the endothelial barrier of cerebral microvessels intact, and that the small quantities of radioactivity detected in the parenchyma in measurements of the brain uptake index essentially represent metabolites due to MAO activity.