he study focused on the evaluation of breeding 150 Sokolsky mares under the genetic resources protection program and used for reproduction within the area governed by the Regional Horse Breeders Association in Bialystok. The analysis took into account the effect of age and origin after father on the values of particular indicators characterizing the reproductive performance of the studied mares. The following reproductive indicators were calculated: mating efficiency, fertility, non-heat percentage, proportion of dead births, and sterility, as well as the length of pregnancy and gestational interval. In total, 849 pregnancies and 700 gestational intervals were analyzed. The average mating efficiency of mares in the study period was 89.59%, fertility 84.53%, and the non-heat percentage 8.01%. Some variations could be observed in relation to miscarriages; the mean value of the indicator amounted to 1.47%. The share of dead births increased in subsequent years of mares’ reproduction performance, reaching the level from 2.30% to 18.31%. A slight increase in the period of pregnancy in older mares and a prominent increase in the length of the gestational interval was reported. The gestational interval ranged from 107.33 days for below 6-year-old mares to 160.94 days at 14-year-old mares. Mares’ origin after different father groups and mating with stallions from different genealogic lines also exerted some impact on the value of these indicators.