The aim of the research was to evaluate the influence of sex, breeding group, litter size and age of lambs when placed in lambing pens for their growth rate during rearing. The average daily gain of lambs was evaluated based on their weighing data each week after their placement into lambing pens. Lambs of Improved Valachian, Tsigai and Lacaune breed (n=26) had been gradually integrated to the experiment. Artificial rearing was carried out using a feed slot machine. All lambs were weighted on a digital scale with an accuracy of 0.1 kg at birth, when placed in the lambing pens and then every week until weaning. The breeding group factor had no statistically significant effect on the average daily gain during milk rearing. The average daily gains of lambs at the end of milk rearing were the same for all experimental grups. The ewe lambs at the end of milk rearing reached a higher average daily gain than ram lambs; but we did not detect statistically significant influence of sex on the average daily gain. The litter size factor had a highly significant effect (P<0.01) only on the growth rate of lambs during the first week after beieng placed in the lambing pens. At the end of milk rearing, the average daily gain was the same for single-litter-lambs and twin-litter-lambs too. The difference of the age of their placement into lambing pens had no statistically significant effect on the weight of the lambs at the end of milk rearing. Statistically significant effect (P<0.05) was found in lambs of different age at their placement in the lambing pens on the average daily gain from placing in to the end of milk rearing. The lambs involved in rearing had a lower weight gains when placed into the lambing pens at an older age. Manuscript profile