Department of Endocrinology, Growth and Reproduction, Erasmus University, Faculty of Medicine, Rotterdam, The Netherlands (Received 1 March 1978) In several avian species the ability of exogenous oestrogen to induce nest-building and in some instances courtship behaviour after ovariectomy is significantly enhanced by exposing the birds to long environmental photoperiods (Steel & Hinde, 1972; Liley, 1976). The same lighting stimulus will also induce ovarian activity in intact birds and a related phenomenon has also been reported in the sheep (Raeside & McDonald, 1959; Fletcher & Lindsay, 1971). In the studies on sheep, it was reported that ovariectomized ewes primed with progesterone displayed sexual behaviour in response to treatment with oestradiol benzoate more readily during the normal breeding season, when days were short, than at other times of the year. This communication reports a failure to obtain comparable results in another seasonally breeding mammal, the ferret (Mustela furo). Female ferrets were purchased