Maternal hyperphenylalaninemia was induced by feeding excess L-phenylalanine to pregnant rhesus monkeys. Animals which received the phenylalanine supplement throughout the entire pregnancy gave birth to infants with low birth weight. Marked elevations of phenylalanine and tyrosine were consistently observed in maternal sera; the levels of phenylalanine and tyrosine in umbilical cord sera were greater than the corresponding maternal value in eight of nine pregnancies. Maternal and umbilical cord levels of serine were slightly elevated and maternal levels of 3-CH3-histidine were reduced in all pregnancies. Values for the other amino acids, and the cord: maternal ratios for all amino acids, including phenylalanine and tyrosine, were within the control range. Infant monkeys born to mothers with hyperphenylalaninemia demonstrated a significant reduction in learning behavior. This experiment indicates that not only is maternal hyperaminoacidemia reflected in the free amino acids of fetal blood, but an added insult is also produced by a normal placental process which functions to maintain higher levels of each of the free amino acids in the fetus than in the maternal organism.