I have an 8 year old pure chihuahua and he has been peeing in the house alot lately. I take him for a long walk before I go out as he is an inside dog, but when we are home and the back door is open he gets slack and pees in house, also been peeing on my dads pillow and on chairs. Theres is about three spots he really pees on, how do I stop him peeing inside???

First of all, has he been checked out by your vet? He might have prostate problems, or an infection.

Has anything changed in your household that has upset him? Even the slightest change can upset a dog. Has he been upset by change in the environment? A new dog in the area could also upset him.

If none of these are the cause of him peeing in the house then I would limit where he can go until you can stop him peeing.

My elderly bitch started peeing in the house for no apparent reason so we did tests at the vets. Nothing showed up. She was put on a course of antibiotics, just in case there was something we hadn't tested for, and she stopped peeing and is now back to her normal self. :-D

He will return to the same place to pee if the smell is there, use bio washing powder to clean the fabric or odour eliminator to get rid of the smells.  Stop access to these areas whilst trying to stop this behaviour.  I would get him vet checked aswell.  Leaving a door open maynot be enough, you may need to take him outside and really praise when he goes.  Have never had a dog revert back to weeing in the house so can only offer the advice I have just given.