Michael Wisher was unavailable to reprise the role of Davros from Genesis of the Daleks, as he was on tour in Australia. David Gooderson had to wear the same mask for that story, which didn't fit his face properly.
This episode contains the notorious moment where the Doctor tells the Daleks to "Back off!". However, Tom Baker messed up the line, resulting in it sounding like "Spack off!" ("spack" is a politically incorrect British slang term for a mentally handicapped person). This quote is known amongst fans as one of the more unintentionally funny moments of the classic series.
This episode was watched by 13.8 million viewers on its original transmission.