What is in a peach pit that makes it fatal to eat?

Cyanide. Peach pits contain trace amounts of a cyanide compound. However in order to do any damage to yourself you would have t eat a lot of peach pits (theoretically). Peaches are not the only fruit to have cyanide related poisons, apple seeds, cherry seeds and nectarines all have these poisons as well, but have much less. Hope this helps.