The human placenta is known to concentrate nearly all amino acids intracellularly for transfer to the fetus. To clarify the mechanism and regulation of this process we have determined the specificity of the principal placental transport systems for neutral amino acids. With the use of competitive inhibition techniques, three transport systems of overlapping specificity have been elucidated. These correspond approximately to the "A", "L", and "ASC" systems of Christensen and associates. In the placenta the specificity of these systems is as follows: A system - alpha aminoisobutyric acid (AIB), glycine, proline, N-methylalanine, alanine, serine, threonine, and glutamine; L system - isoleucine, valine, phenylalanine, BCH, alanine, serine, threonine, and glutamine; and ASC system - alanine, serine, threonine, and glutamine. Placental AIB uptake previously has been shown to increase with preincubation of tissue in vitro. This increase has now been found to be limited to the A system. Activity of the other two systems is essentially unaffected, demonstrating that the transport pathways are separately regulated.