Twenty nine Finland X Rhmani (1/2F.* 1/2R.) male lambs were selected from Sakha Farm. The lambs had average live body weight of 13.9±SE Kg and 70.9±SE days old. The animals were divided into four unequal  groups (8 animals for groups 1, 2, 3 and 5 animals for group 4). After an adaptation period, the lambs were fed the experimental diets, differing in concentrate / forage ratio. Lambs of  group 2 were fed on 100% concentrated feed mixture(CFM), lambs  of group 3 were fed 85% concentrate feed mixture + 15% forage   while, animals of both groups 1 and 4  were fed 70% concentrate feed mixture and 30% forage as the common and control feeding system. All groups received their experimental diets for 68 days after the adaptation period. Three lambs from each of groups 1, 2 and 3 were slaughtered at an average live body weight of 27 Kg. No animals were slaughtered from group 4 since they are similar to those of group 1. The remaining 5 animals from groups 1, 2 and 3 continued to receive the same experimental diets while those of group 4 were switched to receive 100% CFM, until all animals in all treatments reached slaughter weight of about (45kg). Live body weight (LBW) and feed intake were recorded and feed conversion values were calculated during the two stages. Results revealed that average daily gain attained by lambs continuously fed 100% concentrate diet was 272 g, being 216 g in stage 1 and 297 g/h in stage 2.  However, lambs started feeding on 100% concentrate diet after  being fed the control diet, accomplished better growth rate  (324 g/h/d) during stage 2 while was similar to the control one during stage 1 ((187 g/h/d). Lambs fed 85% concentrate (T3) did not show difference than the control. Lambs of both groups fed 100% concentrates either during the whole period or only during the second period reached marketing weight (45 Kg) earlier than the other two treatments by about one month. Feed intake as a percentage of body weight in the first, second and whole stages were calculated and found to range from 3.5 to 3.8 % for lambs. being relatively less than those recommended in NRC (1985) for lambs of daily growth rate from 222 to 324g/h/d. Measured as kg DM/Kg gain, feed conversion (FC)for the comparative trials showed better rate in stage one compared to stage 2 of fattening. In both stages the best FC was attained by T2 which received 100% concentrate diet being, 4.32 and 6.29 kg DM/ Kg BW during stages 1 and 2 of fattening, respectively. The next two parts of this series of experiments will deal with the effects on fermentation in the rumen, anatomy of the digestive system and the histology of the rumen to better interpret the findings before recommending the optimal system of feeding high concentrates.