(TMZ) - Ron Jeremy is in critical condition at an L.A. hospital ... and is being treated for an aneurysm that was located near his heart ... TMZ has learned.

Ron's manager, Mike Esterman, tells TMZ ... 59-year-old Ron drove himself to Cedars-Sinai hospital Tuesday afternoon after experiencing severe chest pain. Docs examined the legendary porn star ... and discovered the aneurysm.

According to Esterman, Ron's condition has worsened and he was transferred to the ICU. He's currently being prepped for surgery.

Ron -- and his 9-inch penis -- have appeared in more than 2,000 X-rated movies together ... including, "Super Hornio Brothers," "High Heels, Low Standards," and "A Christmas Orgy."

He also spoofed Harvey Levin in the XXX spoof, "TMSleaze."