|Abstract:||Early pregnancy loss (EPL) is an important multifactorial condition that affects up to 20% of equine pregnancies, and mare age, parity, previous endometritis and twin pregnancy have all been implicated as causes of early pregnancy loss. The timing of the abortion strongly influences the outcome on the fertility. The regression of the endometrial cups, which form from trophoblastic cells by 35 days of gestation, is a self-regulated mechanism that is independent from fetal death, therefore mares that abort after the endometrial cup formation will present abnormalities in the ovulations, follicular growth and cyclicity due to the concurrent presence of equine chorionic gonadotropin (eCG). Early pregnancy loss has short- and long-term consequences to the horse industry, therefore, the development of novel diagnostic and management tools for early pregnancy loss are warranted. Blood markers and ultrasonographic parameters were assessed in mares undergoing experimentally induced early fetal loss. It was hypothesized that intrauterine infusion of cloprostenol results in earlier fetal compromise than systemic administration. Ovarian structures (number and sizes of follicles and corpora lutea area), fetal heartbeat, and fetal mobility of thirteen singleton pregnancies were assessed daily by transrectal ultrasonography until induction of pregnancy termination (60±2 days of gestation). Mares received 500g of cloprostenol intramuscularly every 12 hours (IM, n=7) or once transcervically (TC, n=6). After initial cloprostenol administration, ultrasonographic examinations were repeated at 6-hours-intervals until loss of fetal heartbeat. Plasma progesterone, estradiol-17β, and alpha-fetoprotein were assessed for five days before and after pregnancy loss. In addition, plasma PGFM concentrations were assessed immediately before cloprostenol administration (0 min), and then 15, 30, and 45 minutes, and 1, 2, 3, 4, 6, 12 hours after administration. Data were analyzed using the MIXED procedure with repeated measures in SAS. Significance was set at P<0.05. All mares lost their pregnancies within forty-eight hours after initial cloprostenol administration, with no difference in time to pregnancy loss. There were significant effects of time starting by 12 h post-induction of pregnancy termination but there was no time by group interaction for progesterone concentrations. Estradiol-17β and alpha-fetoprotein concentrations were not altered upon impending abortion. Concentrations of PGFM increased significantly by two hours after cloprostenol administration, but there were no differences between groups. No time effects or time by group interaction for fetal mobility and heartbeat was detected. Expectedly, the number and area of corpora lutea decreased significantly after cloprostenol administration with no significant differences between groups. In conclusion, intrauterine administration of cloprostenol was not different than repeated systemic administration to terminate the pregnancy. Both models for early fetal loss were equivalent for the endpoints assessed herein. Next, to explore the use of infusions of kerosene to enhance the regression of endometrial cups, thirteen mares had intrauterine kerosene infusions at 21- and 35-days post-abortion. Anecdotally, intrauterine infusion with kerosene has been proposed as a method to treat endometrial cup retention, however there are no controlled studies evaluating kerosene’s usefulness for enhancing endometrial cup regression following abortion. We hypothesized that intrauterine kerosene infusions would hasten regression of endometrial cups without detrimental effects on the endometrium and the mare’s general health. The objectives of this study were to assess the uterine response, systemic side effects and efficacy of intrauterine kerosene infusions to enhance regression of endometrial cups. Nine light-breed mares were enrolled in the study after an experimentally-induced abortion with cloprostenol (n=12) by 60 (602 days) days of gestation. Mares were randomly allocated an intrauterine infusion with 500mL of kerosene (Ker. n=6) or 500mL saline (Cont. n=6) on day 21 and 35 after the pregnancy termination. Uterine biopsies were collected at day 7, 21, 35 and 49, stained with H&E and graded according to the Kenney & Doig 1986 classification. Lymphocyte B CD20, lymphocyte T CD3 and macrophage IBA cell populations were characterized by immunohistochemistry in endometrial biopsies from a subset of mares (n=4/group). Physical examinations, complete blood cell counts, and serum biochemistry were performed before each infusion and then repeated for 2 days after each uterine infusion. Uterine lavage was performed 24 hours after each infusion. Serum samples were collected right before the abortion induction, then at 7, 21, 28, 35, 42, and 49 days after the pregnancy termination for assessment of serum eCG. Continuous data were analyzed with MIXED procedure with repeated measures in SAS, categorical data with LOGISTIC procedure in SAS. Significance was set at p<0.05. Kerosene infusion did not affect complete blood cell counts and serum chemistry parameters. There were no appreciable abnormalities on the physical examinations following kerosene infusions. Concentrations of eCG decreased over time (p<0.001), but there were no differences between groups or time by group interactions (p=0.7128). Histology samples from the uterus showed no signs of increased fibrosis or degeneration in the treatment group. In conclusion, while kerosene infusions did not appear to have detrimental effects on mare health, our findings suggest that the use of kerosene in the uterus does not enhance the regression of endometrial cups.