I think most rats are a bit too big for the common cat, but cats are good and may spook it.
Poison as mentioned above does work, From what I hear. the poison dehydrates the rat, so they try to find water. So if the rat is traped in the house (sort of speak) then it would get caught in a wall. Typically when poisoned they wind up in the basement. I remember we found a dead 8 inch rat in the kitchen closet when I was a kid.
I would suggest the nonkilling traps, they are like racoon traps but smaller. the only problem with them is where do you put the rat. If you put it outside, It would proboly make its way back in. and then some towns have laws that prohibit transporting wildlife (and rodents). so you may want to give your local animal controle a call to check on the laws and options.
Monty
