The changes that occur in the genital organs of the ferret during the oestrous cycle have been described by Marshall [1904, 1933], Robinson [1918], Hammond & Marshall [1930], Parkes [1930] and recently by Hamilton & Gould [1940]. In the present investigation here described our observations were mostly confined to the vulva which from being insignificant during the non-breeding season becomes enormous at oestrus, and is the most obvious indication that a ferret is on heat. Parkes [1930] and Hamilton & Gould [1940] have found that injected oestrogen can more readily induce cornification of the vagina than swelling of the vulva; moreover, it takes less oestrogen to produce changes in the uterus than in the vulva [Parkes, Rowlands & Brambell, 1932]. The ferret normally has two or three annual sexual seasons in the spring and summer, beginning usually early in April and lasting until September, autumn and winter being occupied by