It has been suggested that the conversion of androgens to oestrogens could be involved in the stimulation of oestrous behaviour by androgens in the rabbit (Beyer, Vidal & Mijares, 1970). Recently, it has been reported (Meyerson & Lindström, 1968) that the antioestrogen MER-25 inhibits oestrous behaviour in spayed rats treated with oestradiol benzoate and progesterone. This indicates that MER-25 also competes with oestrogen centrally. We therefore decided to study the effect of MER-25 on the induction of oestrus by both oestrogen and androgen in the spayed rabbit. New Zealand White rabbits, ovariectomized at 3 months, were tested when 6 months old both for lordosis and mounting behaviour in five daily successive control tests. The testing procedure has been described in detail elsewhere (McDonald, Vidal & Beyer, 1970). All rabbits that mated or mounted during this period were discarded. Thirty-eight rabbits were thus allotted at random to the treatment groups detailed