The discovery of the powerful synthetic oestrogen diethylstilboestrol [Dodds, Golberg, Lawson & Robinson, 1938] was quickly followed by the demonstration [Noble, 1938a, 1939a] that treatment with this material produces, in rats, the signs of apparent pituitary dysfunction (e.g. atrophy of the gonads and accessory sex organs, and inhibition of growth) associated with excessive treatment with natural oestrogen in this species. Noble [1938b, 1939b], applying the technique introduced by Deanesly & Parkes [1937], showed that the implantation of crystals or tablets of pure diethylstilboestrol could also induce alterations in the rate of body growth, and in endocrine function. The action of diethylstilboestrol with respect to the gonads was to some extent reversible, in that after treatment with the synthetic oestrogen the ovary of the female rat could still respond to the influence of gonadotrophic preparations. Dr A. S. Parkes, F.R.S., suggested to us that a relatively simple