Eight pregnant mares were randomly allotted to receive 100 and 120% of the recommended digestible energy (DE) (NRC, 1978), respectively, for the last 90 d of gestation. During the first 30 d of lactation, all mares were fed the NRC (1978) recommendations for lactation. The 120% DE-fed mares had an advantage of .18 kg (P<. 03) ADG during the last 90 d of gestation. The length of gestation and foal birth weight were unaffected by treatment. During 30 d postpartum, 120% DE-fed mares lost .16 kg/d while 100% DE-fed mares gained .33 kg/d (P<.04). Postpartum mare ADG were supported by foal ADG, with foals on 120% DE-fed mares having gained .37 kg/d more (P<.06) than 100% DE foals. Pooled data for all mares showed that day to first postpartum ovulation and follicle size on the eighth day postpartum were negatively (r=-.80; P<.05) correlated. First estrus postpartum was negatively correlated (r=-.69; P<.01) to follicle size at the eighth day postpartum and was positively correlated (r=.91; P<.01) to the day of first postpartum ovulation. Serum progesterone (P₄) and luteinizing hormone (LH) values were obtained at 3 d intervals starting 3 wk prepartum through 30 d postpartum with daily values 1 wk pre- and postpartum. No treatment differences within period in hormone concentrations were seen. However, the curvilinear effects of day for LH concentration up to and including the fifth order (r²=.73) and the seventh order for P₄ concentrations (r²=.59) were highly significant (P<.01). All mares received 100 µg gonadotropin releasing hormone (GnRH) on d 6 postpartum. Seven of 8 mares exhibited an LH response (P<.01) within 6 h post-administration. The 120% DE-fed mares experienced an advantage of weight gain during gestation and their foals gained faster during 30 d postpartum. No treatment differences were found for pre- and postpartum hormone concentrations or for the responsiveness of the pituitary as determined by GnRH administration. / Master of Science