The relationship between follicle vascularity and establishment of pregnancy was studied in 21 mares. An ovulation-inducing injection of hCG was given when the preovulatory follicle was 34.0 to 37.0 mm (Hour 0). Mares that had not ovulated by 30 h after treatment were bred once (Hour 30). Each mare ovulated by 48 h after treatment, and 14 mares became pregnant and 7 were nonpregnant. The preovulatory follicle was evaluated by B-mode and Doppler ultrasonography at Hour 0 (before treatment) and Hour 30 (before breeding). B-mode echogenicity and thickness of the stratum granulosum and prominence of the anechoic band beneath the granulosum increased in both pregnant and nonpregnant groups (hour effect, P < 0.001) with no group effect or interaction. An increase in follicle diameter and percentage of follicle circumference with colorDoppler signals was greater between Hours 0 and 30 in the pregnant group than in the nonpregnant group (interactions, P<0.001). Spectral-Doppler measurements were made at the most prominent intraovarian color signal. Decreases in resistance and pulsatility indices were greater between Hours 0 and 30 in the pregnant group than in the nonpregnant group (interactions, P < 0.05), indicating increased vascular perfusion downstream from the spectral measurement in the pregnant group. Relative peak systolic velocity and time-averaged maximum velocity of blood flow at the point of spectral assessment showed a group effect (P < 0.05; greater in pregnant group) without an hour effect or interaction. Results supported the hypothesis that greater blood flow to the preovulatory follicle is associated with higher pregnancy rate.