Having been in the pest management business for over 20 years, I can
tell you that those ultrasonic rodent repellers are a waste of money.
Our industry has done extensive testing and has found no real evidence
of effectiveness.  (If it did, the industry would encourage it's use).

As for repellents, there are none.  As was mentioned in MMD 020811
yesterday, exclusion is best, but when that is impractical, next best
is capturing the rodent very soon after getting in.  Use a rodent
glueboard with a few shelled peanuts in the middle.  The boards are
commercially available at most any hardware store and are non-toxic
and easily disposed of.

Live traps are also good, but are best if baited first with peanuts.

Mike Frost
Manassas, VA