The 2 safe gases for ferret anesthesia are Iso or Sevo.  (Sevo is slightly
safer.) I would avoid the injectable options and just mask them down with
Iso or Sevo then maintain him on Iso or Sevo.  (A third gas is Deso, but
it is currently too expensive for use in veterinary medicine.)
 
Happy Holidays,
Jerry Murray, DVM
[Posted in FML issue 3641]