One thing the GF and I learned is, if they're in the walls/ceiling/rafters, you don't want to use rat poison. They'll take it back up into their nests where they hide (and where you'll never find them) and die there.. Then you'll be stuck with the smell of rotten, decomposing rats stinking up the entire building.
We ended up moving from that house because it got so bad...
First, figure out where their entry and exit points are. Usually a hole somewhere Use old school mechanical rat traps with a little bit of peanut butter around and along the path they take. Here's the important part; before you take the traps out of the package, put on some vinyl or latex gloves before handling and setting the traps. The rats/mice are smart, and can smell the human touch on things, and will avoid the traps if they smell you on it.
Good luck!
interesting 
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