Experiments were conducted to determine whether endogenous progesterone regulates synthesis and/or secretion of luteal oxytocin (OT). In experiment 1, mature ewes (n=5 per group) were assigned randomly to control or mifepristone (RU 486) treatment groups. Ewes were injected twice daily s.c. with vehicle or 10 mg RU 486 from days 5-7 of the estrous cycle (estrus=day 0). On day 8, following an i.v. prostaglandin F₂α (250 μg cloprostenol) challenge, venous samples were collected at frequent intervals to determine plasma OT concentrations. Plasma OT in RU 486-treated animals did not differ significantly from those of the control animals (P>0.05). In Experiment 2, ewes were injected s.c. daily with vehicle or 175 mg RU 486 from days 2-5 of the estrous cycle followed by a prostaglandin F₂α (250 μg cloprostenol) challenge on day 6. Four of five RU 486-treated ewes exhibited "split-estrus" (estrous behavior through 36 hours and again 84 to 108 hours after the onset of initial estrus). There was no significant difference in mean plasma OT or progesterone levels between treatment groups (P>0.05). Mean mature corpus luteum (CL) weights of control and RU 486-treated ewes on day 6 did not differ (394.8 ± 28.8 vs. 319.5 ± 48.3 mg; P>0.05). Mifepristone-treated ewes contained mature CL, new CL (2 of 4 ewes), and/or preovulatory follicles (≥ 10 mm, 2 of 4 ewes). Average interestrous interval for RU 486-treated ewes was 9 days longer than that of control animals (26.2 ± 2.9 vs. 17 ± 0.5 days; P<0.025). A subsequent study was conducted to determine the effects of gonadotropin-releasing hormone (GnRH)-stimulated release of luteinizing hormone (LH) on luteal OT and progesterone production in beef heifers. Ten heifers with normal estrous cycles were assigned randomly in equal numbers to a control and treatment group. On day 2 of the estrous cycle (estrus=day 0) heifers were injected with either physiological saline or 100 pg GnRH every 4 hours for 56 hours. Samples were collected 0 min pre- and 180 min post-GnRH challenge for progesterone analysis. Sixty hours after the initial injection of GnRH or saline, heifers were challenged with an i.v. injection of 500 pg prostagland in F₂α (cloprostenol) and blood was collected at frequent intervals for OT analysis. Luteal OT synthesis was suppressed (P<0.01) in heifers receiving repeated injections of GnRH compared to saline-treated control animals. Progesterone secretion was significantly greater in saline-treated animals compared to GnRH-treated animals pre- and post-challenge (1.0 ± 0.06 vs. 0.93 ± 0.11 ng/ml and 1.16 ± 0.05 vs. 0.96 ± 0.13 ng/ml, respectively; P<0.05). / Graduation date: 2003