Diameter deviation during a follicular wave is characterized by the continued growth of the developing dominant follicle and reduced growth and regression of the subordinate follicles. This study considered the hypothesis that reduced blood flow in the future largest subordinate follicle precedes the beginning of diameter deviation. The hypothesis was tested by quantifying the daily changes in blood-flow velocities and blood-flow area within the wall of follicles before and during diameter deviation in mares (n = 7). The blood-flow end points were quantified daily by transrectal color Doppler ultrasonography. Follicles were identified retrospectively by rank as F1 (largest) and F2 according to the maximum attained diameter. Follicles were grouped into nine F1 diameter ranges of 3.0 mm each (equivalent to 1 day's growth) centered on 6.5, 9.5, 12.5, 15.5, 18.5, 21.5, 24.5, 27.5, and 30.5 mm. Diameter deviation began in the 24.5-mm group, as indicated by a smaller (P < 0.05) difference between F1 and F2 in the 24.5-mm group than in the 27.5-mm group. Based on a similar approach, peak systolic velocity and time-averaged maximum velocity of blood flow began to deviate between F1 and F2 in the 18.5-mm group (P < 0.04) and blood flow area began to deviate in the 21.5-mm group (P < 0.009). Thus, differential blood flow area between F1 and F2 began an average of 3.0 mm (equivalent to 1 day) and differential blood-flow velocities began an average of 6.0 mm before the beginning of diameter deviation. The results demonstrated that deviation between F1 and F2 in the blood flow of the follicle walls occurred 1 or 2 days before deviation in follicle diameter during follicle selection in mares.