One page 173 of Kim Schilling's excellent book, Ferrets for Dummies,
there is a paragraph discussing the drinking of urine by ferrets.  I
will not quote her directly, as the book is copyrighted.  In sum, Ms.
Schilling states that urine drinking is a harmless and rather normal
behavior for some ferrets.  No one seems to know why they do this.
 
Cordially,
Greta Hoisington
 
[Moderator's note: I agree it's *usually* harmless, but it may also be
indicative of a problem.  For example, animals with diabetes often lick
their urine because the excess sugar in the blood is dumped into the
urine making it sweet.  I suggest monitoring for changes.  BIG]
[Posted in FML issue 4332]