NAD+ deficiency has recently been linked with increased occurrences of congenital abnormalities and embryonic death in human and animal subjects. Early embryonic death is a major component of pregnancy loss in mares and very little is known regarding the requirement for NAD+ in horses. The aim of this study was to quantify NAD+ and its metabolites in the plasma and urine of mares after orally administering an acute dose of nicotinic acid and determine the absorption, metabolism and excretion of this essential precursor for NAD+ biosynthesis. Nicotinic acid (5 g per os) was administered to four mares via a dosing syringe. Blood samples were collected at 0, 0.25, 0.5, 1, 2, 4, 6 and 22 h, and urine samples were collected at 0, 3, 6 and 22 h...Nicotinic acid was rapidly absorbed into peripheral blood within 15 min of administration and the concentrations of nicotinic acid, nicotinamide (NAM), nicotinuric acid, nicotinic acid mononucleotide and nicotinic acid adenine dinucleotide (NaAD) increased significantly in plasma at 30 min. The concentrations of NAM, nicotinic acid riboside and NaAD increased significantly in urine at 3 h. The levels of NAM and NaAD remained significantly elevated in plasma at 22 h, sixfold and ninefold greater, respectively, than the basal levels at 0 h. While the extracellular levels of NAD+ in the samples remained undetected, the large, sustained elevation of NaAD levels in plasma indicates that the NAD+ levels were boosted within the cellular compartments. The results show that nicotinic acid supplementation increases the bioavailability of NAD+ precursors in mares, which is proposed to be beneficial during periods of peak NAD+ demand, such as during early embryo development.