A cat using the toilet rather than a litter box seems like something only seen in movies, and that is indeed where the idea originated from. It is something that takes patience and several weeks to accomplish, but it can be done. Some cats even learn to press the handle to flush after using the toilet.
Pros and Cons
The pros of having a cat who uses the toilet rather than the litter box are not having to scoop a litter box each day, and saving money by not buying cat litter.
The cons include a higher risk of injury when a cat jumps onto the toilet. Although it is unlikely that the short fall from the toilet would cause injury, if your cat lands the wrong way, or jumps up when you've removed the toilet tray, whether a commercial tray or one made of a cardboard box, for cleaning, he could land inside the toilet.
Preparation for Training
To begin the process of getting your cat to use the toilet rather than a litter box, replace the traditional plastic box full of litter with a cardboard box full of shredded newspaper. Place this box in the same location as the litter box. After a few days, move the box closer to the toilet. If it is not already in the bathroom, move it closer to the bathroom until it is in the room; begin moving it closer to the toilet. Only move the box 1 to 2 feet at a time. If you move it too far at one time, your cat may become confused and defecate in the original spot, outside of the box. Wait at least a day between each move. It is also a good idea to cut down the height of the box a small amount after a few moves, until it is about 1/2 high. Keep extra boxes of similar size on hand in case the box gets soggy.
Transitioning to the Toilet
Once the box is right next to the toilet, begin raising it a few inches at a time. You can use anything you want to raise it off the floor, just ensure that it is a stable surface that the box cannot fall from. If it does not feel stable, your cat will not use it. Continue lifting it a little at a time until it is level with the toilet seat. Leave it in this position for a week or two to give your cat a chance to adjust to jumping up to use the box. Once your cat is accustomed to jumping up, cut a small hole in the center of the box, about 3 inches wide, surrounding it with shredded paper. Cut the sides off the box and move the box to the toilet. Place the cardboard under the toilet seat to hold it in place and gradually start placing less and less paper in the box and enlarging the hole until it is as big as the hole in the toilet seat. Once you have done this, you can remove the cardboard completely as he will be used to the large hole in the toilet.
There are also commercial products available that work in much the same way as the cardboard box. Most commercial products use a plastic tray and a small hole in the middle. Each week, one ring of the plastic tray is removed, until the tray is removed completely and he is using the toilet on his own.