Diploma thesis is concentrated on analysis of selected factors affecting gestation length in mares of warmblood breed Old Kladruber horse. Gestation length was significantly affected by the month of conception, the month of foaling and the year of conception. The age of mares in the time of conception or foaling did not influence gestation length. Even the sex of foals or alive and dead foals did not significantly affect gestation length. In conclusion, gestation length in mares is affected by a large number of factors and not all of those have been discovered yet. The knowledge of these factors is very important for the most accurate determination of parturition date.