The voluntary feed intake, body composition and efficiency (ME intake/energy retention) of 10 Dorper x Merino and 10 Afrino x Merino lambs were studied. The age of experimental animals ranged from weaning at approximately 120 days, to about 240 days of age. The lambs were fed individually on a diet with a metabolizable energy content of 10,26 MJ/kg. Measurements were made continuously of livemass, voluntary intake and body composition using tritium dilution. The growth results were analysed and interpreted relative to percentage of mature mass to account for differences in size. The intake of the Afrino x Merino cross was the highest and the intake of rams was superior to that of ewes. The Afrino x Merino cross had the highest growth rate. It was also clear that the Afrino x Merino cross gained protein and fat at a faster rate than the Dorper x Merino cross. The efficiency, as defined here, was very similar in sex groups. It was concluded that in spite of mature mass as a scaling factor, body composition and voluntary feed intake were a major source of variation in determining efficiency.