We recently reported an unusual abundance of arginine (4–6 mM) in porcine allantoic fluid during early gestation. However, it is not known whether such high concentrations of arginine are unique for porcine allantoic fluid or whether they represent an important physiological phenomenon for mammals. The present study was conducted to test the hypothesis that arginine is also the most abundant amino acid in ovine allantoic fluid. Allantoic and amniotic fluids, as well as fetal and maternal plasma samples, were obtained from ewes between Days 30 and 140 of gestation. Glycine was the most abundant amino acid in maternal uterine arterial plasma, representing approximately 25% of total α-amino acids. Alanine, glutamine, glycine, plus serine contributed approximately 50% of total α-amino acids in fetal plasma. Fetal:maternal plasma ratios for amino acids varied greatly, being less than 1 for glutamate during late gestation, 1.5–3 for most amino acids throughout gestation, and greater than 10 for serine during late gestation. Marked changes were observed in amino acid concentrations in amniotic and allantoic fluids associated with conceptus development. Concentrations of alanine, citrulline, and glutamine in allantoic fluid increased by 20-, 34-, and 18-fold, respectively, between Days 30 and 60 of gestation and were 24.7, 9.7, and 23.5 mM, respectively, on Day 60 of gestation (compared with 0.8 mM arginine). Remarkably, alanine, citrulline, plus glutamine accounted for approximately 80% of total α-amino acids in allantoic fluid during early gestation. Serine (16.5 mM) contributed approximately 60% of total α-amino acids in allantoic fluid on Day 140 of gestation. These novel findings of the unusual abundance of traditionally classified nonessential amino acids in allantoic fluid raise important questions regarding their roles in ovine conceptus development.