Short-term fasting of mature ewes during diestrus results in increased serum concentrations of progesterone and a delayed pre-ovulatory surge release of LH. To determine if these changes in reproductive hormones influence subsequent follicular development, mature ewes observed in estrus were assigned randomly to control (n=10) or fasted (n=15) groups. Control ewes had ad libitium access to feed, whereas fasted ewes were not fed from day 7 through 11 of their estrous cycle. Daily blood samples were collected from control and fasted ewes throughout the fasting period. Fasting increased (P < 0.001) serum concentrations of progesterone (4.4 ng/mL versus 2.7 ng/mL [+/- 0.3]). On day 12, all ewes were treated with 10 mg of PGF(2 alpha) and fasted ewes were returned to ad libitum feed. Ovaries were collected from ewes (n = 5 each group) at 0 and 72h following PGF2a in control and 0, 72 and 96 h in fasted ewes. Ovaries were weighed and small (<= 2 mm), medium (3-4 mm), and large (>= 5 mm) follicles were enumerated. Total numbers of follicles were less (P < 0.001) in fasted than fed ewes (74.6 versus 30.2 [2.2]) at Oh, but did not differ (P=0.9) when numbers of follicles were compared at similar times before the anticipated LH surge (i.e., at 72 h versus 96 h in control and fasted ewes, respectively). Within follicular size class, numbers of small and medium follicles were decreased (P=0.04) at 0 h in fasted ewes. Numbers of large follicles did not differ (P=1.0) between groups. Although numbers of small and medium ovarian follicles in fasted ewes recovered by 96 h to values comparable to fed ewes at 72 h following PGF(2 alpha), serum concentrations of estradiol 17 beta (P=0.08) and FSH (P=0.06) tended to be decreased in fasted ewes before the anticipated surge release of LH. Pituitary content of LH and FSH also tended to be lower (P <= 0.09) at 96 h in fasted ewes than at 72 h in control ewes, but did not differ (P >= 0.4) at hour 0 following PGF(2 alpha). Hypothalamic and stalk median eminence contents of GnRH were not influenced (P >= 0.2) by fasting at any time period. Fasting during the luteal phase perturbs gonadotropin secretion and may influence fertility by causing a delay in ovarian follicle development. (c) 2006 Elsevier B.V All rights reserved.