Infant rhesus monkeys were (a) fed a diet high in phenylalanine or (b) fed a diet high in p-chlorophenylalanine for the 1st 12 months of life, (c) selected from mothers fed a diet high in phenylalanine during pregnancy, (d) fed a diet low in phenylalanine, (e) maintained as controls, or (f) maintained as pair-fed controls. Tested during the 1st year of life with familiar peers, during the 2nd year with unfamiliar stimulus animals and with a movie film, the 3 groups of phenylketonuric monkeys showed less play and other positive social behavior and more withdrawal and more aggressive behavior than did the 2 control groups. The results support learning data suggesting that phenylketonuric monkeys are more emotionally reactive.