Variations in estrous and ovulation times occur because of follicle diameter during PGF2 alpha injection, in mares. The purpose of this study was to see the effect of follicle diameter on estrous and ovulation. Therefore 43 estrous cycles were evaluated in 32 Thoroughbred mares. Ten mg PGF2 alpha (Dinoprost Tromethamine, Dinolytic) was injected intramuscularly in diestrous period for synchronization. Follicle diameters were measured and divided into groups in centimeters as follows less than or equal to 1 cm, 1-2 cm, 2-3 cm, 3-4 cm ve > 4 cm, during PGF2 alpha injection. Result of daily teasing and rectal palpation every other day and daily rectal palpation during estrous period revealed that post injection-estrous. estrous period, post injection-ovulation. and estrous-ovulation times were in group less than or equal to 1 cm 5.91+/-2.71, 7.25+/-1.43. 11.75+/-2.23 and 6.16+/-1.95 days, in group 1-2 cm 4.81+/-2.84, 6.56+/-1.94, 10.68+/-2.95 and 5.87+/-2.85 days, in group 2-3 cm 3.90+/-1.44, 5.10+/-1.57, 8.00+/-1.78 ad 4.20+/-1.53 days and in group 3-4 cm 3.00+/-1.41, 4.60+/-1.35. 6.20+/-2.31 and 3.60+/-1.01 days, respectively. There was no follicle > 4 cm during PGF2 alpha injection. As a result, reduced estrous and ovulation times occured because of enlarged follicle diameters during PGE2 alpha injection. Therefore it was concluded that evaluation of ovaries for follicle structurs before PGF2 alpha injection would be beneficial.