Marie,
Behavioral changes may be just behavioral or caused by any illness that makes
the ferret uncomfortable and less tolerant of others.  The most common
disease that causes return of a neutered male ferret to  male behavior
(mounting females,marking their territory,stronger scent) or aggressive
behavior is an adrenal tumor.  If there are any other symptoms (esp.
hairloss) this is probably the cause and with treatment he will go back to
his good old self.  There is a new bloodtest that is reportably very accurate
to diagnose adrenal tumors.
Although this could be behavioral I would hate to see you getting to mad at
him for a problem he can't control if it is a medical condition.
Dr Weiss  301-299-4142
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[Posted in FML issue 1142]