The present investigation was undertaken to quantitate the whole-blood amino acid concentrations of the mother and fetus over an extended period of fasting using a chronic sheep preparation. Fetal amino acid levels generally increased or remained relatively high during the fast (e.g., alanine, glycine, valine, leucine). In contrast to the situation in the fetus, most amino acids (alanine, glycine, glutamine, serine, and threonine) decreased in concentration in the ewe with fasting. Alanine levels changed most dramatically in the ewe, decreasing to less than 50% of the fed state values within 72 h of fasting. Therefore, while the supply of gluconeogenic amino acids becomes limited within the maternal circulation during fasting, levels of these amino acids remain high or increase in the fetal arterial blood.