Recent work, mainly American, has indicated the possibility of using implanted synthetic oestrogens to induce faster and more economical weight gains in fattening lambs. Little has yet been done in this country to repeat or to extend these observations, and a possible reason for this may lie in the difference between the organisation of the fat lamb industries of the United Kingdom and the United States. Here, as in New Zealand and in other countries rearing sheep principally on pastures, many lambs are sold fat at light weights, while those not so well finished at weaning may either be fattened on grass or other green crops, or be overwintered and fattened as hoggets on grass in the following spring. In the U.K. fat lambs are thus seldom fed dry feeds as they are under U.S. ‘feed-lot’ conditions, and are slaughtered at rather lighter weights than their U.S. counterparts.