Results of the analysis are presented of the effect of inbreeding on selected reproduction traits and
body weight in a Booroola flock kept closed for 14 years at the Institute of Genetics and Animal
Breeding, Jastrzêbiec. The mean inbred in lambs rose linearly until 2002, when it reached the level of 9.81%, ranging from 0 to 31.7%. No inbred effect was observed on ovulation rate, fertility, litter size, frequency of stillbirths and results of rearing lambs (lambs survival rate) until week 4 of age. Negative effect of inbred was identified on body weight at birth and on week 4 and 8 of age.