92 beef synthetic and dairy synthetic bulls were fed low-, medium- or high-energy diets for 140 days. Diet did not significantly affect av. daily gain, but the scrotum and testes of bulls on the high energy diet were significantly heavier than those of bulls on the other diets. Scrotal circumference was higher, and testes and scrotal skin weight were higher in dairy synthetics than in beef synthetics. These and other scrotal and testis traits were not affected by diet. Diet did not significantly affect 9 carcass traits. After weaning 36 bulls were given to appetite for 140 days diets containing hay and grain 90, 65 or 40%. The bulls were slaughtered at average weight 450, 500, 550 and 600 kg. Bulls fed on the high energy diet grew faster and had larger rib-eye areas than the other groups. Larger bulls had proportionally larger testes and scrotal circumference. The effects of breed, Dairy Synthetic and Beef Synthetic, were as pronounced as those of dietary energy. Carcass traits were not affected by diet.