- Sheep are seasonally polyestrus, which means they do exhibit estrus and ovulation during the short daylight (breeding season) and do not during the long daylight (nonbreeding season). To stimulate the ewes to ovulate during the nonbreeding season, various hormonal treatments have been used. PG-600 contains both equine chorionic gonadotropin and human chorionic gonadotropin. It is used off-label by U.S. sheep producers to induce an ovulatory estrous cycle during the nonbreeding season. However, PG-600 use has been associated with an increased number of tertiary follicles and reduced pregnancy rates in sheep. To investigate these reproductive effects, two experiments were performed to compare two dosages of PG-600 during breeding and nonbreeding season. Following treatment with intravaginal progesterone and cloprostenol, ewes during the breeding season were treated with 5 mL PG-600 (T1; n=8), 1.5 mL PG-600 (T2; n = 8), or 5 mL saline (C; n = 8) and then mated to rams. Jugular vein samples were collected prior to the PG-600 injection (0 hr) and at 2, 4, 6, 8, 12, and 24 hr after injection. Serum estradiol-17β and progesterone was determined by chemiluminescence and levels among groups were analyzed statistically using repeated measures analysis of covariance. Ovulation and pregnancy rates were determined by transrectal ultrasonography and compared by one-way ANOVA and chi-square, respectively. During the breeding season, estradiol-17β concentrations were greater in T1 compared to T2 and C (P < 0.001). Also, serum progesterone concentrations were significantly different among groups over time (P < 0.00001). In addition, ovulation rate was greater (P < 0.001) but pregnancy rate was lower (P < 0.001) in the T1 compared to C and T2. However, during the nonbreeding season, serum estradiol concentrations, ovulation, pregnancy, and fecundity rates, and weaning weights were not significantly different among groups; although serum progesterone concentrations increased in T1 compared to C and T2 (p<0.00001). These data confirm that a 5 mL dose of PG-600 administered to ewes during the breeding season (but not the nonbreeding season) increases estradiol concentrations and lowers pregnancy rates. To follow up on this result, the effects of PG-600 on endometrial estrogen receptor concentration [ESR] and follicle stimulating hormone receptor (FSHR) expression during the breeding season were investigated. The same experimental design described above was used with the omission of the 1.5 mL PG-600 group. Four and seven days after PG-600 (T1) or saline treatment (C), six ewes from each treatment and control group were generally anesthetized, and a laparotomy was performed to obtain endometrial samples. Tissue samples were collected ipsilateral to the ovary bearing the highest number of corpora lutea. Immunohistochemistry and an estradiol exchange assay were used to determine the pattern of FSHR immunoexpression and [ESR], respectively. Two-way analysis of variance was used to determine the effect of treatment. Significance was defined as p<0.05. FSHR expression increased four days after PG-600 treatment but decreased seven days following PG-600 treatment. Endometrial [ESR] did not differ by treatment but did differ between day four and day seven in the PG-600 group. These data suggest that reduced pregnancy rates observed following PG-600 treatment in ewes are not related to alterations in [ESR] but may be related to alterations in FSHR. Further research is needed to determine if alterations in endometrial FSHR affect endometrial mucin 1 expression and/or myometrial contractility, which would negatively impact early embryo survival.