Prostaglandin F2 alpha was commonly used for synchronisation in mares. The purpose of this study was to see the effect of single and double doses of PGF2 alpha on estrous, ovulation and pregnancy rates. Therefore 43 Thoroughbred mares, between 4-17 years old, were used. Thirty mares 10 mg PGF2 alpha (Dinoprost Tromethamine, Dinolytic) was injected intramuscularly, regardless of the stage of the estrous cycle, for synchronization. Estrous behaviors and follicular development were examined with daily teasing and rectal palpation every other day, after PGF2 alpha injection and ovulation times determined daily rectal palpation during estrous. The mares which in estrous (n: 11) natural service or artificial insemination with fresh semen were done every other day until estrous behavior lasted. The mares (n: 19), were not estrous after first PGF2 alpha injection, were injected second PCF2 alpha 14 days later and repeated all applications after first PGF2 alpha injections. Control group (n: 13) were chosen from the mares which were not injected PGF2 alpha in estrous and natural service or artificial insemination with fresh semen were done every other day until estrous behavior lasted. For pregnancy diagnosis, rectal palpation were done all mares between 30-35th and 60th days after ovulation. Estrous, ovulation and pregnancy rates obtained from first or second PCF2a injected mares and control group were 36.66%, 33.33% and 45.45%; 68.42%, 78.94% and 46.66; 100%, 92.30% and 53.84%, respectively.