Studies that have investigated the effects of L-arginine supplementation on blood flow to the uterus have not determined the optimal time of supplementation. No research has been conducted on nitrite concentration in the placenta and blood of mares supplemented with L-arginine. L-arginine is a precursor of NO and therefore, should increase nitrite production both in blood as well as in expelled placenta. Presence of nitrites would also be indicative of increased angiogenesis. No studies have addressed placental efficiency in mares supplemented L-arginine. Placental efficiency is vital for production of a healthy foal. Therefore, to address these short-comings the objectives of this study were to supplement pregnant mares with L-arginine on effects of supplementation at differing time points on blood flow to the uterus, nitrite concentration in blood and placenta, and placental efficiency.