Diets containing altered levels of specific amino acids or their metabolites were fed to rhesus macaques during infancy and during fetal life. Learning tests, after the animals were placed on a normal diet, indicated permanent mental retardation in monkeys that received phenylalanine either prenatally or for 3, 6, or 12 months postnatally, and also in monkeys fed parachlorophenylalanine, which inhibits the metabolism of phenylalanine. Retardation was not detected in monkeys fed diets high in other amino acids or in pair-fed control animals.

Infant Macaco arctoides were reared under standard conditions. Changes in body weight, body length, head circumference, hematologic status and serum proteins during the first year of life were defined in relation to the ad libitum consumption of a standardized milk diet, and compared to those derived from infant M. mulatta. Both sexes consumed a comparable volume of diet per kilogram body weight, but male M. arctoides infants were significantly larger than females in all body dimensions : this finding was not noted in infant M. mulatta. The body weight and head circumference of infant M. arctoides were greater and their body length was less than that of M. mulatta.

1. Serial butanol extractable iodine (BEI) determinations were performed on the sera of fifty-one healthy pregnant and non-pregnant female monkeys (Macaca mulatta) over a period of 18 consecutive months.2. The mean serum BEI value for non-pregnant monkeys was 3\m=.\5\ m=+-\ 0\m=.\8 \g=m\g./100 ml. Monthly variations from the mean were not significant. Each animal demonstrated serial BEI values about a mean specific for that animal.3. An apparent rise in BEI values was observed 22-35 days following conception and continued during the first 120 days of pregnancy; thereafter, values tended to remain level until shortly before term when they dropped slightly. A transient elevation was frequently observed during the first 5 days following delivery, after which the values dropped rapidly and reached control levels by 35 days post partum.