The Brigadier's line "Well, here we go again" up on seeing The Doctor's regeneration was ad-libbed by Nicholas Courtney.
Since this story and the following story Robot were being filmed concurrently Jon Pertwee's regeneration wasn't the last scene he filmed, but Tom Baker requested that the regeneration be his first scene, recalling the scene at the 50th anniversary convention he mentioned that Jon said he'd buy Tom a pint, something that he never did.
Episode 6 is the first time the term "regeneration" is used to explain the Doctor's change of appearance. Previously his first change was called a "renewal" with the Doctor claiming it was the TARDIS that made it possible. The second was forced upon him by the Time Lords as part of his exile to Earth. Any reference was always referred to as a "change of appearance" as the concept of regenerating being a natural part of the Time Lord race had not yet been established.
This episode was watched by 8.9 million viewers on its original transmission.