During the recent years, mares have been shown as relevant follicle-related research comparative model for women because of similarities in the number and nature of ovarian follicular waves, a constant relative diameter of the largest follicle between the two species at definable events throughout the ovulatory wave, and similar ultrasonographic characteristics and changes of the preovulatory follicle. In the mare, as in other monovular species (cattle, women), usually only one dominant follicle develops as a result of the deviation mechanism. However, occasionally two or more dominant follicles may also occur. Few studies in mares have addressed the relationships between periovulatory circulating hormone concentrations and single versus multiple dominant/preovulatory follicles. Temporal and mechanistic studies performed recently (years 2005 to 2008) have contributed to elucidation of intriguing relationships among the dominant follicle and circulating hormones and will be the focus of this review. The main topics discussed herein are: (i) development of one versus two dominant follicles with a single ovulation; (ii) development of one versus multiple ovulatory follicles; (iii) conversion of two dominant follicles to double ovulations; (iv) role of hormones in development of double ovulations; (v) interrelationships of periovulatory reproductive hormones; (vi) repeatability of preovulatory follicle diameter and hormones; and (vii) factors that affect preovulatory follicle diameter and hormones, such as breeds and types of mares, season, body condition, and aging.