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1. Seventy-two bull calves, comprising forty-eight Ayrshire and twelve each of the Friesian and Jersey breeds, were used in a randomized block experiment. They were reared from birth on liquid diets, offered in two feeds daily, at either restricted or ad lib. levels. The restricted level of feeding was such that sufficient diet was fed to allow for a weight gain of I kg/d, and the calves given this level of feeding were slaughtered at a weight 77 kg above the mean birth weight of the breed. The calves given the diets ad lib. were slaughtered at 22 % of mature cow weight of the breed.2. Three milk substitutes, based on spray-dried skim-milk powder, spray-dried whey powder and margarine fat, were compared. Two of these were high-protein diets (26?29 % protein) designed to contain either 20 % fat (LFHP) or 30 yo fat (HFHP) and the other was a low-protein diet (19 % protein) designed to contain 30 yo fat (HFLP).3. Age at slaughter was lowest for diet HFHP and highest for diet HFLP. Total dry-matter intake did not differ between treatments, but daily dry-matter intake was lower on diet HFLP. Relative weight gain did not differ between diets LFHP and HFHP but was much greater than that for diet HFLP. After adjustment for differences between treatments in mean daily dry-matter intake, relative weight gain tended to be highest for diet HFHP. No difference occurred between treatments in the incidence of diarrhoea and of a high rectal temperature (> 39.33?). However, mean rectal temperature was higher for calves given the HFHP diet than for those given the HFLP diet.4. Digestibility and nitrogen and calcium balance trials were made on six of the replications of Ayrshire calves at 4 and 10 weeks of age. Dry-matter intake at 10 weeks of age was lower for calves given diet HFLP than for those given the other diets. Apparent digestibility of fat tended to be lower for the HFLP diet. True digestibility of protein did not differ significantly between treatments. Apparent digestibility of lactose was highest for the HFLP diet.5. No difference occurred between the two high-protein diets (LFHP and HFHP) in N or Ca retention, but N and Ca retention by calves on the low-protein diet (HFLP) was markedly lower. Daily faecal N excretion was unaffected by treatment, but daily urinary N excretion was lower and the biological value of the protein was higher for diet HFLP than for diet LFHP, with the corresponding values for diet HFHP being intermediate between the two. N reten tion/100 g weight gain was lower on diet HFLP, lower at 4 weeks of age than at 10 weeks, and lower at the restricted than at the ad lib. level of feeding at 4 weeks of age. Metabolic faecal N and endogenous urinary N at 4 weeks of age were estimated as 0.192 g/100 g dry-matter intake and 192.9 mg/kg0.72live weight respectively.6. Haematological findings showed that the high-fat diets (HFHP and HFLP) resulted in a slower rate of decline in packed cell volume and haemoglobin content with age. The low protein diet (HFLP) resulted in a lower rate of increase with age in the percentage of lymphocytes. Diet HFHP resulted in a decline with age in the percentage of eosinophils compared to a marked rise with the other two treatments.7. Carcass weight did not differ between treatments, but perirenal fat deposition was much higher for the calves given the high-fat diets (HFLP and HFHP).8. It is concluded that dietary fat is not a readily available source of energy for increasing N retention in calves given large quantities of milk substitutes, based on milk products with added fat. Unless additional fat deposition in the carcass is required, no advantages in weight