AbstractBackgroundDysregulation of the estrogen receptor 1 (ESR1) expression during the establishment of pregnancy could contribute to reduce fertility reported in ewes treated with PG‐600.ObjectivesThe objective of this study was to evaluate the effect of treatment with PG‐600 on ESR1 expression in the ovine endometrium during early diestrus.MethodsPolypay ewes (n= 24) that had exhibited an oestrous cycle of normal duration (16–18 days) were treated with intravaginal progesterone‐releasing device (CIDR) for 9 days. Cloprostenol was administrated two days before CIDR withdrawal. On the day of CIDR withdrawal (day 0), ewes received a 5 mL intramuscular injection of PG‐600 (400 IU equine chorionic gonadotropin and 200 IU human chorionic gonadotropin) or saline. Blood samples were collected before treatment and on the day of tissue collection to determine serum estradiol‐17β and progesterone concentrations using radioimmunoassay. On days 4 and 7, six ewes from each treatment group were anaesthetised and a laparotomy was performed to obtain intercaruncular endometrial samples. Endometrial samples were collected ipsilateral to the ovary bearing the highest number of corpora lutea. An estradiol exchange assay was used to determine ESR1 concentration.ResultsEstradiol concentrations did not differ by time or treatment, but progesterone concentrations were significantly higher in the PG‐600‐treated group on day 7 despite a similar number of corpora lutea. Endometrial ESR1 concentration was significantly reduced on day 7 in the PG‐600 group.ConclusionsIn conclusion, although progesterone was higher than controls on day 7, ESR1 did not differ from controls suggesting that treatment with PG600 is unlikely to impair reproductive potential.