Check out the Rat Zapper....

http://www.ratmousezapper.com/_inv/ultra.html

I have the Rat Zapper 2000.... had a minor infestation a couple years ago
through the dryer vent. Over 24 hour period, I killed 10 rats/mice. They
tell you that you may need to leave it in place for 2-3 days before mice
will be brave enough to enter. In my case, they ran into it within an hour.
It only kills one at a time, but it is very very nice in that you don't
have mangled bodies laying around and you can simply dump the dead rat into
the trash, pop a few chunks of dog food into it, set it down and leave it
for a while.

Found mine at Harbor Freight on sale for $30 and quite happy with it.
Neighbor used it to get rid of squirrel from his attic.

Oh.. word of advice. If you dump a dead rat into the trash, warn the wife.