Domestic Animal Endocrinology 29 (2005) 573–581 Follicular fluid concentrations of free insulin-like growth factor (IGF)-I during follicular development in mares L.J. Spicer ∗ , C.A. Santiago 1 , T.R. Davidson, T.S. Bridges, C.S. Chamberlain Department of Animal Science, Oklahoma State University, Stillwater, OK 74078, USA Received 8 December 2004; received in revised form 24 February 2005; accepted 16 March 2005 Abstract The objective of the present study was to evaluate changes in concentrations of free insulin-like growth factor (IGF)-I in follicular fluid (FFL) during follicle development in the mare. Mares (n = 14) were classified as either in the follicular phase (n = 8) or luteal phase (n = 6). Follicles (n = 92) were categorized as small (6–15 mm; n = 54), medium (16–25 mm; n = 23) or large (>25 mm; n = 15) and FFL was collected. Free IGF-I levels in FFL in large follicles of follicular phase mares were greater (P < 0.05) than in large follicles of luteal phase mares and small or medium follicles of luteal and fol- licular phase mares. Free IGF-I concentrations were greater (P < 0.05) in large follicles of luteal phase mares than small but not medium follicles of luteal phase mares. FFL ratio of estradiol:progesterone paralleled changes in free IGF-I. Free IGF-I concentrations were negatively correlated (P < 0.05) with insulin-like growth factor binding protein (IGFBP)-2, -4 and -5 but not IGFBP-3 levels. In addition, free IGF-I concentrations in FFL were positively correlated (P < 0.01) with FFL estradiol, proges- terone, androstenedione, estradiol:progesterone ratio, total IGF-I and total IGF-II. We conclude that increases in intrafollicular levels of bioavailable (free) IGF-I are associated with increased steroido- genesis in developing mare follicles. © 2005 Elsevier Inc. All rights reserved. Keywords: Insulin-like growth factor; Follicle; Follicular fluid; Mare ∗ Corresponding author. Tel.: +1 405 744 6077; fax: +1 405 744 7390. E-mail address: igf1Leo@okstate.edu (L.J. Spicer). 1 Present address: Institute of Animal Science, University of the Philippines, Los Ba˜ nos, Laguna 4031, Philippines. 0739-7240/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.domaniend.2005.03.003