Castoreum is the exudate from the castor sacs of the mature North American beaver (Castor canadensis) and the European beaver (Castor fiber).
Castoreum is the yellowish secretion of the castor anal sac. Beavers use castoreum in combination with urine to scent mark territory. Both beaver sexes have a pair of castor sacs and a pair of anal glands, located in two cavities under the skin between the pelvis and the base of the tail. The castor sacs are not true glands (endocrine or exocrine) on a cellular level, hence references to these structures as preputial glands, castor glands, or scent glands are misnomers.
It is used as a tincture in some perfumes and as a food additive.