The interovulatory intervals of mares contain various combinations of minor follicular waves (largest follicle does not become dominant) and major waves (largest follicle becomes dominant). After emergence of an ovulatory follicular wave, the follicles grow in a common-growth phase until the beginning of deviation. At deviation, the dominant follicle continues to grow and the subordinate follicles begin to regress. Deviation begins when the future dominant follicle is about 22.5 mm. The capacity for follicle dominance is similar among the four largest follicles at the beginning of deviation, as indicated by a study involving ablation of one, two, or three largest follicles. The ovulatory waves, as well as major anovulatory waves and minor waves, originate from the stimulation of an FSH surge, which reaches a peak when the largest follicle is about 13 mm. The initial decline in the FSH surge appears to be a function of inhibin; circulating estradiol does not begin to increase until about 2 days after the FSH peak or about 1 day before the beginning of deviation. Concentrations of LH of the ovulatory LH surge reach a transient plateau encompassing deviation of the ovulatory wave. The intrafollicular concentrations of estradiol, IGF-1, inhibin-A, and activin-A increase differentially in the future dominant follicle versus the future subordinate follicles about 1 day before the beginning of diameter deviation. These factors may be enablers for differentially enhancing the FSH and LH responsiveness of the future dominant follicle, based on the results of in vitro studies in nonequine species. Injection of a physiologic dose of IGF-1 into the secondlargest follicle of mares at the expected beginning of deviation increased the concentrations of inhibin-A and activin-A, but not estradiol, within 24 hours. Injection of an IGF binding protein into the largest follicle at the expected beginning of deviation resulted in decreases within 24 hours in several follicular-fluid factors in the largest follicle and ovulation from the second-largest follicle. Ablation of the largest follicle at the expected beginning of deviation resulted in experimental deviation between the two largest remaining follicles beginning 24 hours after ablation; concentrations of IGF-1 increased differentially in the converting future dominant follicle 12 hours before the beginning of experimental deviation, whereas inhibin-A, activin-A, and estradiol did not begin to increase differentially until 24–48 hours after the beginning of deviation. Results of these three experimental approaches indicated that the IGF-1 system is critical for the initiation of deviation in mares.