The penis sheath of adult Fallow deer, which is an integral part of the abdominal skin, extends from the scrotum to the preputial orifice. Marked seasonal changes occur in the prepuce. In the non-rutting animal, the outer region of the prepuce, which is similar in structure to normal skin, passes through a transition zone characterized by a corrugated epidermis and multi-lobed sebaceous glands, before becoming the inner-sheath lining. Just before the rut, hypertrophy of the epidermis of the transition zone occurs with the formation of keratinous villi-like processes. The sebaceous glands enlarge and lipid production is increased. At the beginning of the rut, the transition zone everts and becomes located on the outer periphery of the prepuce, which now has the appearance of a dark brown, coarse, short-haired brush. At this stage, the animal and its voided urine develop a characteristic pungent odour whereas urine aspirated from the bladder is relatively odourless. After the rut, the prepuce returns to normal, the keratinous papillae being shed. It is suggested that the strong smelling rutting odour of male Fallow deer helps to synchronize oestrus in the females. The gross hyperkeratosis and associated epidermal hypertrophy of the prepuce produces a sponge-like mass which may help in scent production and dispersal.