l-Norepinephrine and serotonin have been regarded as preferential substrates for monoamine oxidase (MAO) type A. A close comparative examination of a number of tissues from different species, however, indicated the following differences. Serotonin was a more selective substrate for MAO-A, being inhibited by low concentrations (less than 10(-7) M) of the irreversible MAO-A inhibitor, clorgyline, more consistently and to a greater extent (80-100%) than was l-norepinephrine (30-85%). These serotonin-norepinephrine differences were greater in humans and other primates than in rodents. Serotonin also had a 2- to 4-fold smaller apparent Km for MAO-A than l-norepinephrine and was deaminated 2- to 5-fold more readily by MAO in vitro in most tissues. In contrast, the MAO-B in human platelets deaminated l-norepinephrine more readily than serotonin. Thus, l-norepinephrine, like dopamine, should be regarded as a substrate for both MAO-A and MAO-B in vitro. The prominent role of MAO-B in norepinephrine degradation in primates may need to be considered in interpreting laboratory and clinical studies of clorgyline and other selective MAO-inhibiting drugs.