Twelve Mutton-Walrich Merino cross wether lambs were randomly allotted to two groups of 6 at 8 weeks of age after they were weaned. The one group received a high energy diet just below ad libitum and the other 80% of this calculated ad libitum until 36 weeks of age. Intake and mass gain of all lambs wore measured every week, while body fat and protein were derived from fortnightly measurements of tritiated water space. Although the ad libitum group consumed more and gained more in body mass, fat and protein until 36 weeks of age, there were no differences when the two groups were compared within the same body mass interval. The results between 15 and 45 kg body mass for the ad libitum and 80% ad libitum groups were respectively: ME required: 1 881 ± 129 and 1 884 ± 219 MJ, mass gain; l74 ± 25,5 and 157 ± 18,5 g day-1,fat gain: 7,84 ± l,6l and 6,68 ± 1,43 kg, protein gain: 3.90 ± 0,51 and 4,00 ± 0,69 kg and days required:1 76 ± 31,5 and 193 ± 21,0. None of these differences were significant at the 5% level of probability. Yet, it was observed that the ad libitum group tended to deposit more fat and less protein than the 80% ad libitum group. In terms of fat or protein deposition per unit mass gain it was in fact found that the ad libitum group gained consistently more fat and less protein than the 80% ad libitum group, although the data was only statistically significant (p ˂0,05) above 35 kg body mass. It was concluded that this change in body composition was responsible for the fact that the 80% ad libitum group exhibited the same efficiency (MJME kg-1mass gain) as the ad libitum group.