The subcutaneous implantation of stilboestrol or hexoestrol has been shown to increase the daily rate of live-weight gain and efficiency of food utilization in lambs (Andrews, Beeson & Harper, 1949;Jordan, 1950 (1956) found that the carcasses from treated steers contained more muscle, more bone and less fat than carcasses from untreated animals. O'Mary et al. (1952) reported that, whereas the subcutaneous fat of treated lambs was thinner and contained more moisture, there were no significant differences in muscular development or in intermuscular fat. Whiting, Clark & Allen (1954) suggested that the increase in live weight of treated lambs might be due entirely to an increase in moisture content of the tissues and to a higher percentage of offal.These conclusions have been based on limited carcass measurements and analyses of small sample joints such as the cut incorporating the ninth, tenth and eleventh ribs in steers (Aitken & Crichton, 1956) and the eleventh rib in lambs (O'Mary et al. 1952). Since stilboestrol is known to alter the conformation of the animal (Acker, Whiteman, Gallup & Tillman, 1955) the composition of such sample joints is unlikely to be representative of the carcass as a whole. Moreover, the claims that stilboestrol treatment improves the efficiency of utilization of food are based only on live-weight increases per unit of food consumed and can have little significance until the composition of the extra live weight of treated animals is known.The object of the present experiment was to determine the composition of the weight gains of lambs implanted with hexoestrol and the relative efficiency with which the lambs converted protein and energy in the food to protein and fat in the carcass.Thirty-six Suffolk x Half-bred, 5-month-old, wether lambs were allocated at random to three groups. The first group, designated the ' sample-slaughter group', was killed at the beginning of the experiment. Each lamb of the second group had implanted subcutaneously in the left ear a 15 mg pellet of hexoestrol. The third group was a control. Implanted and control lambs were housed individually indoors and were given a daily ration of 100 g hay and increasing amounts of a mixture of 8 parts maize meal, z parts crushed oats, 2 parts bran, I part white-fish meal and I part linseed cake for an experimental period of about 90 days.After 50 days it was no longer possible to feel the pellet in the ears of the animals and a further implantation of a 15 mg pellet of hexoestrol was made.The lambs given hexoestrol and the controls were slaughtered at a similar stage of fatness as decided by two independent judges. One animal from the control group and one from the implanted group failed to adjust itself to indoor feeding conditions and the results for these two lambs have been excluded.Slaughter technique. Immediately after slaughter the warm carcass weight of each lamb was recorded together with the weights of the head, feet, skin and fleece, pluck (i.e. heart, lungs and liver), kidney and associated fat and the ali...