Isoflurane, an inhalant.  Dr. Bruce Williams, DVM, says:

    The only acceptable type of anesthetic agent for general
    anesthesia in the ferret is gas, and preferably a gas anesthetic
    called isoflurane.  Most vets use it, but other types of gas
    anesthetics, such as halothane are still in use.  Isoflurane
    currently is the safest, with the least chance of generating a
    life-threatening cardiac arrhythmia or causing liver disease, both
    of which may be seen (rarely) with halothane.  Most
    ferrets, even with severe disease, will go down quickly with
    isoflurane, and come up within 5-10 minutes.  No other
    premedications are necessary [unlike for the injectable ketamine].

    I would not use a vet who used injectable anesthetic for surgery -
    chances are much higher for overdosing.  The effects of injectable
    anesthetics are extremely unpredictable in the ferret, and older
    ferrets are at risk for arrhythmia and cardiovascular shock.