Revolution at the 5 to 15 pound cat size is what is used when it is  
also providing protection from heartworm.

These past vet posts will help people:
http://ferrethealth.org/archive/FHL5195
(from 9 days ago)
>The Idexx SNAP heartworm test (ie "the dog test") can be used in
>ferrets. However the Idexx well test that Idexx runs when you send
>blood samples into an Idexx lab is even more sensitive and accurate.
>Thus the best test is to have your vet send a blood sample into an
>Idexx lab for a heartworm test. If it is positive, then a treatment
>protocol (ie ivomec and pred or proheart 6) can be used. Remember
>heartworms in ferrets is usually fatal, so prevention is your best
>option. You can use Revolution, Advantage Multi, Interceptor,
>Heartgard, Proheart 6 or ivomec to prevent heartworms. Heartworms
>cause damage to the heart and when they die, they cause damage to
>the lungs. Both can be fatal to a ferret.
>
>Hope that helps,
>Jerry Murray, DVM

http://ferrethealth.org/archive/FHL5194
(10 days ago)
>Yes, the dose for heartworm prevention is one full tube of the cat
>size Revolution (5-15 lb cat). IF you just want flea and ear mite
>treatment, then you can use one full tube of the kitten size
>Revolution (0-5 lb cat). This is based on a small study that Pfizer
>did with Revolution several years ago. Colorado State University did
>the study with Revolution for ear mites (works well, but you have to
>treat every animal in the house). Pfizer in Australia also did a small
>study with Revolution for heartworm prevention and Auburn University
>also did a small heartworm study with Revolution. Thus it has been
>tested and is safe and effective for heartworm prevention, but they
>recommend one full tube of the cat size Revolution once a month.
>
>Hope that clears things up,
>Jerry Murray, DVM

and there are more...

Proheart, the absolutely BEST medicine for ferret who are already
infected with heartworm is being re-introduced after careful studies
showed the risk it posed to certain specific individual dogs was not
at any unusual level for such meds.

You can read about just why Proheart is so important for saving ferret
lives here:

http://www.miamiferret.org/fhc/heartworm.htm

and you can tract down later info by Dr. Cottrell in

http://weah4.tripod.com/id22.html

There are other meds as well, each with their own strengths. In

http://ferrethealth.org/archive/FHL353

notice which one is only "sort of safe". It has increased risks so
is not as safe a choice for ferrets. Notice also in the archives the
ferrets who have been killed by over the counter parasite products.

Sukie (not a vet)

Recommended ferret health links:
http://pets.groups.yahoo.com/group/ferrethealth/
http://ferrethealth.org/archive/
http://www.afip.org/ferrets/index.html
http://www.miamiferret.org/fhc/
http://www.ferretcongress.org/
http://www.trifl.org/index.shtml
http://homepage.mac.com/sukie/sukiesferretlinks.html

[Posted in FML 6007]