pure peppermint oil. I read online that rats hate it. I went to GNC purchased a small bottle for 5-6 dollars. Put it on cottonballs and placed them down into the rat's holes. That was 2 weeks ago and I have not seen ANY indication that they are coming around. I was seeing little feet prints in the mud, poo, etc. now nothing. I would also occasionally cover their holes and they immediately would dig them out again.......nothing! Hopefully that did it and safely for my ducks.