EN
During the period 1997-2010 the development trends in reproductive traits of selected prolific breeds of sheep (Finn, Friesian, Olkuska and Romanov sheep) were studied. During that period it was found that the number of ewes of Olkuska and Romanowska breeds, that were evaluated according to their performance, increased, while the number of Finn and Friesian sheep decreased. It has been shown that the fertility of evaluated ewes, except for the Olkuska breed, deteriorated. The ewe prolificacy deteriorated, the only exception was the Finnish sheep; however, the positive trend was observed only in the period 1997-2003. The positive developmental trend of the lambs rearing was recorded only for the Friesian breed. In terms of reproductive performance, the negative trend was observed for all breeds. Deterioration of the reproduction rates of prolific breeds in the successive years, may adversely affect the profitability of sheep production, and consequently, lead to the liquidation of flocks.