"Friends" star Matthew Perry died with significant levels of the drug ketamine in his blood stream.

While Perry was receiving prescribed ketamine from a psychiatrist as part of his therapy for depression ... it was pretty clear that Perry was getting large additional quantities of the anesthetic drug.

And that's what the DEA and LAPD are investigating.  But how does someone get their hands on illicit ketamine?

KNX chief correspondent Charles Feldman put that question to Dr. Adam Pruett, he's a psychiatrist practicing in Vermont ... and was one of the early adopters of ketamine for therapeutic purposes.