Reproductive performance is the largest determinant of income in the livestock industry. In the U.S. sheep industry, embryonic and fetal deaths during pregnancy account for almost half of the total number of fertilized ova and a majority of these losses have been reported to occur before d 18. In study 1, the objective was to determine if arginine supplementation enhances ovarian function and prevents early reproductive losses in sheep. Ewes received L-arginine HCI (equivalent to 27 mg of L-arginine/ kg of BW, ARG, n = 20) or saline (CON, n 20) i.v. from d O (estrus) to d 15. On d 12, serum concentrations of arginine (nmol/ml) were elevated in ARG vs. CON ewes at O (P < 0.001 ), 0.5 (P < 0.001 ), 1 (P < 0.001 ), 2 (P < 0.005 ), and 4 h (P < 0.05), but were similar (P > 0.05) at -0.5, 8 and 24 h. Pulsatility index in the ovarian artery on d 12 was reduced in ARG vs. CON ewes (P < 0.05). Despite similarities in the number of corpora lutea (CL) per ewe (P > 0.05), ARG ewes had greater P4 concentrations throughout treatment compared to CON ewes. Although pregnancy rate was not influenced (ARG, 55% and CON, 60%; P > 0.05), ARG ewes had more embryos per ewe (P:::; 0.04) and less CL not represented by embryos (P '5 0.03) compared to CON ewes at d 25 of pregnancy. Ewes treated with ARG gave birth to more lambs when compared to control ewes (ARG, 1.6 ± 0.16 vs. CON, l. 1 ± 0.16 lambs born per ewe). In summary, early reproductive losses can be prevented by treatment with arginine. The objective of Study 2 was to determine if arginine supplementation surrounding the time of maternal recognition of pregnancy enhances ovarian function and minimizes reproductive losses. Ewes received L-arginine HCl ( equivalent to 27 mg of L- arginine/kg of BW, ARG, n= 47) or saline (CON, n = 47) i.v. from d 9 to d 14 following estrus ( d 0). On d 10, serum concentrations of arginine (nmol/mL) were elevated in ARG versus CON ewes at O (P < 0.001), 0.5 (P < 0.001), 1 (P < 0.001), 2 (P < 0.001) and 4 h (P < 0.001 ). Despite similarities in the number of CL per ewe (P > 0.05), serum progesterone concentration (ng/mL) was greater in CON compared with ARG on d 9 (P < 0.02) and I 0 (P < 0.005). Treatment with arginine influenced pregnancy rate (ARG, 55% and CON, 30%) throughout the treatment period. Ewes treated with ARG gave birth to similar (P > 0.05) number of lambs when compared to CON ewes (ARG, 1. 78 ± 0.17 vs. CON, 1.6 ± 0.27 lambs born per ewe). In summary, arginine supplementation surrounding the time of maternal recognition of pregnancy may prevent early reproductive loss or influence vascular resistance and circulating serum progesterone concentration in ewes. In study 3, the objectives were to determine if rumen-protected arginine supplemented to ewes on d 8 to 13 of the estrous cycle affected serum amino acid concentration, ovarian blood flow, and circulating progesterone. Ewes fed 360 mg/kg BW arginine (360 ARG) had greater serum arginine concentration than control (CON), 90 mg/kg BW arginine (90 ARG), and 180 mg/kg BW arginine (I 80 ARG) on d 11 (P < 0.07) and d 12 (P:::: 0.03 ). Arginine supplementation increased peak systolic velocity in the CL for 360 ARG and 90 ARG compared to CON (P < 0.04). Supplemental rumen-protected arginine had no effect on serum concentration of progesterone (P > 0.50). Results indicate that rumen-protected arginine supplemented to ewes at the rate of 360 mg/kg B W may increase circulating serum arginine concentration, in addition to increasing ovarian blood flow.