The aim of this study was to investigate the effect of single dose dinoprost on estrus synchronization and pregnancy rate in purebred Arabian mares during diestrous. Thirty-five purebred Arabian fertile mares, aged from 5 to 12 years were used between April and May. Animals were divided as treated with dinoprost group (n: 20) and control group (n: 15) Control group consisted of mares that had estrus spontaneously. Treated mares were divided into two groups according to their follicle size (^20 mm (n:9) and <20 mm (n:l 1)) detected by means of ultrasonography during dinoprost injection. The side effect score of dinoprost injection was evaluated clinically. The mares having estrus which were detected by means ol teaser stallion, endometnal edema and follicular growth were bred naturally every other day until ovulation. In the treated mares, estrus rate was 90%. The side effect score due to intramuscular dinoprost injection was 2,45± 0.95. Ovulation and pregnancy rates were 94%. 65%, and 86%, 54%, in treatment and control groups, respectively. There was no statistically significant difference between treatment and control groups for ovulation and pregnancy rates. Dinoprost injection-estrus (day) and estrus-ovulation intervals (day) were 2.0+0.5, 4+1.2 and 4.6± 0,9, 5±1.06 in &20 mm and <20 mm follicle size groups on their ovary during dinoprost injection, respectively. There was a statistically significant difference (p<0.0l) between two follicle size (*20 mm and <20 mm) groups for the mean interval between injection and estrus. As a result, dinoprost synchronized the estrous at a high rate. The side effects such as sweating, colic and ataxia due to dinoprost disappeared within a few hours However, variable ovulations times were obtained and lollicle on the ovaries affected estrus due to dinoprost injection. Fertility was not impaired by this treatment Dinoprost can öe used easily for estrus synchronization