In the wild, the guinea pig does not foul her burrow but relieves herself elsewhere. This makes the guinea pig receptive to litter box training, though it's not a guarantee that your guinea pig will accept it. If you'd like to try training your guinea pig to use a litter box, you need to provide a box. You need one that fits your guinea pig. Check with pet supply stores for litter boxes; you may find boxes made for cats, rabbits, or specially for guinea pigs. Organic, paper litters are best because dusty clay- and wood-based litters can cause respiratory problems.
To begin training, place the litter box in the area of the guinea pig's cage where she normally relieves herself and away from food and water. Place a few fecal pellets in the box. To encourage her, you can place a small amount of hay. Some guinea pigs enjoy sitting in the litter box, munching on the hay. That's okay. Eating stimulates digestion and the guinea pig will use the box as intended. Hopefully, your guinea pig makes a connection and uses the box. When you see your guinea pig using the box, praise her and offer a treat. Never scold or holler at the guinea pig for not using the litter box.
Remember, not all guinea pigs take to litter box training, and being negative certainly won't encourage her. Once the guinea pig uses the litter box in her cage, the next step in training is to get the guinea pig to return to the box when she's outside the cage. This is easiest if you train in one confined area at a time, so place the guinea pig in a small room such as a bathroom or laundry room, along with a litter box, food, water, etc. Watch to see if she returns to the litter box, when nature calls. If she does, offer praise and a treat. If not, return to the cage and begin again.
As the saying says, what goes in must come out. The food the guinea pig ingests will be digested and come out as fecal matter. With the guinea pig, though, the food is digested twice for maximum nutrient absorption. Guinea pigs, like rabbits, produce two kinds of fecal pallets, hard and soft. The soft pallets, which are rich in protein and vitamins, are passed from the anus and eaten by the guinea pig. This unusual process is how the guinea pig gets the most of the hard-to-digest foods she eats. You may notice your pig eating these pallets. Though it is offensive to some owners at first, realize it's healthy and normal for the guinea pig to do this.
Eventually, you can expand the boundaries if the guinea pig consistently uses the litter box. Do this a little at a time, say from the bathroom only, to the bathroom and a hallway. Don't make the mistake of increasing the area too quickly, though, or you'll be cleaning up a mess. As the guinea pig becomes trustworthy in one area, you can increase the boundaries gradually. Some owners avert accidents by using more than one litter box, placing a new box in each area new to the guinea pig.