What kind of stains - urine, blood, other? The unfortunate reality is that wood is porous so whatever it is has probably soaked well below the surface and in the seams a large volume of liquid can even soak down to the sub floor. Sometimes the only option is to have it professionally restored or replaced.
Go to or call someone who installs wood floors. Ask them if there is a cleaner that can be used. Don't try anything till you talk to them. Some of these cleaners do not work if another cleaner has been used.