Анотация: The aim of the study was to assess the weight development of sheep from the North-East Bulgarian Merino breed, with a genetic component of the Australian Merinos ( AM ) and Booroola ( Bo ) breeds. Analyzed were 584 pedigrees and a total of 1002 records for live weight at weaning, at 9 and 18 months of ewes raised in the Scientific Center for Agriculture - Targovishte. An animal model was used to achieve this goal. The established average values for live weight were: 27.227 kg at weaning, 41.473 kg at 9 months and 51.111 kg at 18 months. The variation of the trait in all three ages was within 12 percent. Genetic estimates of the individual generations indicated a decrease in live weight at weaning and at 9 months. The phenotypic ealization of the trait followed a clear negative tendency of the three ages. No genetic trend of dominance for live weight in purebred animals compared to animals with different participation of the AM breed in the genotype has been established. The genetic component of the Booroola breed has a lasting negative effect on the live weight of the studied ages