SRD:Fast Healing

Fast Healing (Ex)
A creature with the fast healing special quality regains hit points at an exceptionally fast rate, usually 1 or more hit points per round, as given in the creature’s entry. Except where noted here, fast healing is just like natural healing. At the beginning of each of the creature’s turns, it heals a certain number of hit points (defined in its description). A creature that has taken both nonlethal and lethal damage heals the nonlethal damage first. Fast healing does not restore hit points lost from starvation, thirst, or suffocation, and it does not allow a creature to regrow lost body parts. Unless otherwise stated, it does not allow lost body parts to be reattached. Fast healing does not increase the number of hit points regained when a creature polymorphs.