The composition of rat placentae or fetuses at Days 14, 17, and 21 of gestation and of 1-day-old pups was determined by measuring water, lipid, nonlipid, inorganic substances, and free amino acids. Placental weight significantly increased (P less than 0.05) from Day 14 to Day 21. The concentrations of amino acids in the placenta were as follows: 1) glutamic and aspartic acids decreased as gestation proceeded; 2) leucine, phenylalanine, tyrosine, and isoleucine reached a minimum on Day 17, while there were no differences between Days 14 and 21; 3) glycine, methionine, and phosphoserine attained a maximum on Day 14, followed by lower and constant concentrations on Days 17 and 21; and 4) serine, lysine, threonine, ethanolamine, valine, arginine, and ornithine remained constant. Fetal weight increased significantly (P less than 0.05) during the last week of gestation; no changes in body weight were observed from Day 21 of gestation to 24 h after birth. Fetal water concentration decreased significantly (P less than 0.05) and nonlipid concentration increased significantly (P less than 0.05) as gestation progressed; after birth, increased lipid and decreased water contents were observed. The concentration of each amino acid in the fetuses increased significantly (P less than 0.005) as gestation advanced except for alpha-aminobutyric acid which significantly decreased (P less than 0.001). There were no differences in the concentrations of amino acids between Day 21 fetuses and 1-day-old pups. The total amount of free amino acids in the rat fetuses increased significantly (P less than 0.001) after Day 17 of gestation. The measured changes in placental weight and composition were not correlated with the observed changes in the fetus. The amino acids measured at term in the rat placentae were present in the same relative amounts as reported for human placentae.