What is it that l-deprenyl (selegiline) might do?

There have been many claims that l-deprenyl may have distinct properties in slowing and perhaps even in reversing the progression of Parkinson's disease and other neurodegenerative conditions. This article will consider the paucity of evidence that such is the case in humans and the more detailed results from studies with experimental animals indicating that deprenyl may indeed express such a property. The conflicting data on its mechanism of action are considered and the concept that it may function to enhance neuronal fitness is advanced as an alternative to the neuroprotection and neurorescue hypotheses. Possible lines of experimental development that would help resolve some of the many unanswered questions regarding l-deprenyl function are outlined.