Three model amino acids resistant to metabolic alteration are, like normal amino acids, strongly concentrated across the placental barrier in the guinea pig. Accordingly these substances are suitable for study of this active transport. Although maternal tissues accumulated these 3 amino acids vigorously, the fetal tissues reached only moderately higher levels of the models than did the fetal plasma. Two fluorine-containing amino acids showed no advantages as models; one of them was only slightly concentrated by the placenta.