In acyclic embryo recipient mares, artificial cycle protocols are conventionally based on estradiol followed by long acting (LA) progesterone injections. The use of intravaginal devices has been encouraged because it allows a rapid resynchronization of recipients (Rigoleto-Júnior et al., Journal of Equine Veterinary Science 2021; 97:103325) and days of estradiol injections have also been discussed (Silva et al., Journal of Equine Veterinary Science 2021; 103:103669), but further studies are needed to develop more appropriate protocols. The present work aimed to evaluate factors that affect fertility in a commercial ET program. A total of 114 healthy mares (5-15yr-old) were used as embryo recipients and divided into two groups according to ovarian activity: cyclic (n=57) and acyclic (n=57, anestrus or transitional). Day-0 was defined as the day of ovulation in the cyclic group and the day of a 1g intravaginal P4 device (iP4) insertion in the acyclic group. Before iP4 insertion, 17β-estradiol doses varied from 0 to 10mg daily depending on the presence of natural or induced uterine edema that ranged from 3 to 7 days in duration. A one-by-one Pearson's Chi-square test was used to compare pregnancy rates between groups and the effects on pregnancy rate of 1) number of days of 17β-estradiol injections, 2) uterine edema (0-4 score)on D0 in the acyclic group, and 3) day of ET in both groups. Plasma P4 concentrations were evaluated daily by RIA in acyclic (n=10) and cyclic (n=13) mares from D0 to D7. Repeated measure models followed by Tukey's test were used to compare mean plasma P4 within groups. Pregnancy rates did not differ between acyclic (82%, 47/57) and cyclic (75%, 43/57) mares (P=0.5). Uterine edema on Day 0 (P=0.7) and day of ET (P=0.05) did not affect pregnancy rates of acyclic mares. Three days of estradiol treatment in acyclic mares resulted in a lower pregnancy rate (P=0.0425) than four days of injections. For cyclic recipient mares, day of ET affected pregnancy rates (P=0.0001) where D4 (96%, 26/27) showed higher pregnancy rates than D5 (66.7%, 12/18) and D6-7 (41.7%, 5/12), but D5 and D6-7 did not differ. Mean plasma P4 concentrations varied over days for both groups (P>0.05). In cyclic mares, mean plasma P4 increased daily to D5 (plasma P4 = 10ng/mL) followed by a plateau, whereas in the acyclic group, iP4 insertion caused an abrupt rise in plasma P4 concentration within 24h (plasma P4=5.67mg/mL) followed by a gradual daily fall to D7 (plasma P4=2.78mg/dL). In conclusion, the use of acyclic recipient mares prepared with iP4 results in similar pregnancy rates as in cyclic mares despite differences in the plasma P4 profile.