Releasing Fancy Mice
If you have a fancy mouse (a mouse that was bred to be a pet, from a pet store or breeder) don't even consider releasing it in the wild. It will die very quickly because it doesn't have the correct genetic traits for wild survival. Wild mice have developed genes that help them survive. Even with this advantage only 5% of all wild mice survive the first year. Only the smartest, fastest, strongest, healthiest and luckiest survive to reproduce and pass on their genes. Your fancy mouse will not be among these luckiest few because the traits that make a good pet aren't the same as those that help a mouse survive in the wild. If you need to give up your fancy mouse, consider the Humane Society or check out the forums on my links page. You may find someone in your area to take your mouse as a pet.
Releasing Pets of Wild Stock
What about releasing a pet from wild stock? I've done this once and would only do so again after careful consideration. I've released 2 brothers that were raised in captivity, but were only slightly tamed. One of them, Gordo, was very aggressive and didn't get along with any other mice except his brother Mr. Earl. I considered keeping him alone or even with Mr. Earl, but he just didn't seem very happy in captivity even though he had never known anything else. I tried to integrate him with other mice so he would have more company. This looked promising when I got a larger cage, but eventually he killed one of his cage mates. Because of his aggressiveness and the fact that he was a deer mouse, I didn't think there was much chance of finding him an alternate home. I decided that the best thing was to release him. He was 7 months old and even though he hadn't been raised in the wild, I thought his age would give him some advantage over other wild mice. I also decided to release his brother, Mr. Earl, with him. My thinking was that they would stand a better chance of survival together.
I'm sure Gordo was better off in the wild. He looked like he was having a great time and if he lasted a week, I'm sure it was the best week of his life. Releasing Mr. Earl was a big mistake. He seemed to be confused and frightened when I released him. I would have re-caught him, but he was too wild and wouldn't let me get close enough to catch him. I monitored them for a week and then lost track of them.
Relocating Wild Mice
If you caught a wild mouse and want to relocate it, a little extra consideration can help it survive in its new surroundings. I assume you have some concern for the mouse's welfare or you wouldn't be reading this. These are my suggestions and I don't expect everyone to be as concerned as I might be. After reading this you can decide how much effort you're willing to put into relocating.
One thing most people don't consider when they catch a mouse is that it may be a female and have a nest full of nursing babies. Nursing moms are usually much more agitated and aggressive about trying to escape than other mice, so they can get back to their babies. If you relocate a nursing mom, they babies will die in the nest. You can often tell if a mouse is nursing by looking for protruding nipples. Usually you won't be able to see the nipples unless she is nursing. I would release a nursing mom to return to her babies and try to re-catch her and the babies in a few weeks and than relocate them all together.
Relocate the mouse as soon as possible. If you can't relocate the mouse immediately, avoid handling it; wild animals do best when they have little human contact. If the weather is extremely cold I would wait for better conditions. A relocated mouse will have no shelter or food stores to help it through bad weather conditions.
In order to keep the mouse from returning, you should take it about 2 to 3 miles from your house. Try to find a location away from human habitation that provides some shelter like a pile of wood, branches or rocks, or at least some low ground cover. This will shelter the mouse from predators until it can build a proper nest. I would also leave some food (a quarter cup of uncooked oat meal, peanuts, seeds or even dry dog or cat food would be fine) and some nesting material (a few torn up paper towels), this is more important in colder weather. Hide the food where larger animals can't reach it. Dropping it into the sheltering pile of rocks or sticks is a good way to prevent birds and squirrels from eating it. The mouse will come back to collect and hide it for later consumption.
I know this sounds like a lot of work just to release a wild mouse. Living in the wild is always precarious. Release in the wild with no food stores or handy shelter can change precarious to death. If you care enough to relocate, a little extra effort will help make it successful. This is what you don't want to do:
Hawk snatches released mouse
YouTube video by marcel tasch.