Relationship Between Follicle Size and Concentrations of Steroids and Insulin-like Growth Factor (IGF)-I in Mares L.J. Spicer, T.R. Davidson, C.S. Chamberlain, and M.E. Payton Story in Brief The objective of this study was to determine if stage of the estrous cycle or follicle diameter influences concentrations of steroids or IGF-I in follicular fluid of mares. Follicular fluid and granulosa cells from small (<15 mm), medium (16-25 mm) and large (>25 mm) follicles were collected from luteal (n=6) and follicular (n=8) phase mares. Follicular fluid estradiol concentrations differed with estrous cycle stage and follicle diameter. In contrast, progesterone levels were not affected by either estrous cycle stage or follicle diameter. Concentrations of IGF-I did not differ with estrous cycle stage, but increased with follicle diameter. These results indicate that estrous cycle stage and(or) follicle diameter influence concentrations of steroids and IGF-I in follicular fluid of mares. Key Words: Follicular Growth, Mare, Estradiol, Follicular Fluid, Insulin-like Growth Factor Introduction In the mare, one follicle is selected from a cohort of follicles to become dominant. After selection, the dominant follicle continues to grow until ovulation, while the remaining cohort, or subordinate follicles become atretic and regress (for review see Ginther, 2000). Limited information is available regarding the physiological mechanism of follicle selection and maturation in the mare. During preovulatory follicular development in the estrous mare, follicular fluid IGF-I levels increase (Spicer et al., 1991), but changes in follicular fluid IGF-I over a wider range of follicle sizes in the mare have not been reported. Therefore, the specific objectives of the present experiment were to determine whether stage of the estrous cycle or follicle diameter influences concentrations of steroids and IGF-I in follicular fluid of mares. Materials and Methods In late May a total of 28 ovaries were obtained at a commercial abattoir from 14 mares of various breeds, ages and sexual maturity. The mares were classified as either in the luteal (n = 6) or follicular (n = 8) phase based on gross ovarian morphology; ovaries with a viable (vasculature visible) corpus luteum (CL) were classified as being in the luteal phase, and ovaries with large follicles and a corpus albicans or regressing CL (as indicated by pale color and little or no vascularity with a diameter < 20 mm) were classified as being in the follicular phase. Follicular fluid from individual follicles were collected separately using needles and syringes. After granulosa cells were separated from follicular fluid by centrifugation (220 x g for 5 to 7 min), individual follicular fluid samples were frozen at –80 °C. Concentrations of progesterone (P 4 ) and estradiol (E 2 ) in follicular fluid were determined with a double-antibody radioimmunoassays (RIA) as previously described (Stewart et al., 1996; Bridges et al., 2002). Concentrations of androstenedione (A 4 ) in follicular fluid were determined using