The relationship between changes in IV tyramine pressor sensitivity accompanying selective monoamine oxidase (MAO) inhibitor treatment and estimates of MAO-A and MAO-B inhibition in vivo were studied. Reductions in platelet MAO activity provided an index of MAO-B inhibition, while changes in plasma 3-methoxy-4-hydroxyphenethylene glycol (MHPG) were used as an hypothesized reflection of MAO-A inhibition. Chronic treatment with the MAO-A inhibitor clorgyline and the MAO-B inhibitor pargyline showed significant inhibition of the alternate MAO enzyme as well, although this crossover effect was greater for pargyline than clorgyline. The MAO-B inhibitor deprenyl appeared to maintain the greatest degree of MAO inhibition selectivity in vivo. Tyramine pressor sensitivity changes accompanying administration of the MAO inhibitors were highly correlated with decreases in plasma MHPG (r = 0.92), supporting our previous data indicating the rank order of clorgyline greater than pargyline greater than deprenyl for enhancement of tyramine pressor sensitivity and, thus, suggesting that tyramine potentiation is primarily a function of MAO-A rather than MAO-B inhibition. Changes in plasma MHPG are suggested to provide a potentially useful clinical index of in vivo MAO-A inhibition.