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 [log in to unmask] [Posted in FML issue 1142]