Monoamine oxidase inhibitors (MAOIs) are not only effective antidepressants, but also have several other applications. Yet, they are infrequently used as a result of their potential to cause toxic side effects such as tyramine-induced hypertensive crisis (cheese effect). A resurgence of interest in MAOIs followed the finding of two forms of MAO. This resulted from the development of drugs that selectively inhibit the metabolism of serotonin and norepinephrine (MAO-A) or dopamine and phenethylamine (MAO-B). MAO-B is the predominant MAO found in the human brain. Theoretically, selective MAO-B inhibition can enhance brain MAO levels while leaving intestinal MAO-A intact, thus bypassing the cheese effect, L-deprenyl is the most extensively studied MAO-B inhibitor. At low doses, it is very selective for MAO-B and is not associated with the cheese effect. At higher doses, it is virtually nonselective. L-deprenyl enhances the effect of L-dopa on Parkinson's disease and may retard its natural progression. Although studies have found L-deprenyl to be an effective antidepressant only at nonselective doses, certain subtypes of depression may respond to selective doses. Also, evidence suggests that L-deprenyl has a positive effect on the general function and cognitive abilities of Alzheimer patients. Studies to date, including those showing a significant increase in the life span of rats following L-deprenyl use have led to the speculation that L-deprenyl may not only treat or retard degenerative diseases and acute brain insults, but may prove to be the first antiaging medication. Several other potential applications of MAO-B inhibitors include panic, ADHD, sexual dysfunction, and PTSD. It remains unclear what role MAO-B inhibition plays in the various therapeutic effects of L-deprenyl. Other potential mechanisms are discussed.