Follicular Wave 1 emerges near ovulation and Wave 2 emerges during mid-diestrus and are stimulated by FSH Surges 1 and 2, respectively. During a follicular wave, one follicle becomes dominant and continues to grow, and the others become subordinates. The eminent and recognizable event during this selection process is diameter deviation. The relationships between the patterns of the wave-stimulating FSH surge and deviation were studied by altering the FSH surge, using follicle ablations. Codominance was used as the primary indicator of altered deviation. In waves with codominant follicles, the day of deviation was based on the beginning of regression of the third-largest follicle. The following groups (n=8/group) were used: 1) controls, 2) 6.5-mm group; all follicles ≥ 5.0 mm ablated except F1 (largest) and F2 when F1 was ≥ 6.5 mm, 3) Day-4 group; all follicles ≥ 5.0 mm ablated on Day 4 (ovulation=Day 0) and 4) Day-7 group; all follicles ≥ 5.0 mm ablated on Day 7. Ablations were done during the decreasing FSH concentrations in Surge 1, during the minimal FSH concentrations between Surges 1 and 2, and during the increasing FSH concentrations in Surge 2 in the three ablation groups, respectively. Concentration of FSH at the beginning of deviation was 38% greater (P < 0.03) in the 6.5-mm group than in the controls. The greater FSH concentrations did not interfere with deviation, as indicated by the unaltered interval from emergence to deviation and an unaltered incidence of codominant follicles ≥ 10.0 mm. Ablations on Day 4 induced the most prominent surge as indicated by a 74% greater (P < 0.003) FSH peak and 31% greater (P < 0.05) FSH concentration at deviation than in controls. The Day-4 group had a greater (P < 0.05) rate of codominance (6 of 8 waves, 75%) than in each of the other groups (25%). The day at the beginning of deviation between F1 and F3 in codominant waves was not different from the day of deviation between F1 and F2 in single dominant waves. Ablations on Day 7 induced a surge with a peak intermediate between the peak for the control and Day-4 groups (P < 0.01) but with no difference from controls in FSH concentrations near deviation and in the rate of codominance. There was no difference among groups in the length of the interval between wave emergence and the beginning of deviation, despite the experimentally increased FSH concentrations encompassing deviation in the 6.5-mm and Day-4 groups. This result indicated that the initiation of deviation was more dependent on attained follicle diameters than on the concentrations of FSH reached during the decline in the FSH surge. Follicle ablation during the low FSH concentrations between surges induced the most prominent FSH surge and was the only group with altered deviation as indicated by a high incidence of codominance