—l-phenylalanine (1 mg/g body wt) or physiological saline (0.9% NaCl) was given intraperitoneally to infant (7-day old), immature (14-day old), and adult (42-day old) rats. The state of ribosomal aggregation was determined in the cerebral postmitochondrial supernatant and purified polyribosome fractions prepared in the presence of rat liver ribonuclease inhibitor. Polyribosomes isolated from cerebral cortices of infant and immature rats 30 or 60 min after administration of phenylalanine were partially disaggregated, whereas the state of aggregation of polyribosomes from mature cerebrum was unchanged. In contrast, little or no evidence of phenylalanine-induced polyribosome disruption was noted in the postmitochondrial supernatant fractions, from which the cerebral polyribosomes were prepared, in any of the animals. Omission of the ribonuclease inhibitor resulted in polyribosome disaggregation in the postmitochondrial supernatant fractions prepared from saline-treated as well as phenylalanine-treated infant rats, but the disruption was more profound in the latter group. Ribonuclease activities in cerebral postmitochondrial supernatant preparations from infant and immature rats were higher than the corresponding values in preparations from adult animals. In addition, the administration of phenylalanine resulted in increases in ribonuclease activities in cerebral postmitochondrial supernatant preparations from the younger animals, but had no effect on these activities in adult animals. These results suggest that alterations in structure and function of polyribosomes from the infant rat cerebrum following a loading dose of phenylalanine were related to exposure of the polyribosomes during isolation to elevated activities of cerebral ribonucleases resulting from this treatment. This hypothesis was supported by the finding that phenylalanine treatment had no effect on the incorporation in vivo of intracisternally-administered radioactive lysine into total, soluble or ribosomal protein of infant cerebrum. However, when cerebral ribosomal RNA was differentially labelled in phenylalanine-treated and saline-treated infant rats by the intracisternal administration of [3H] or [14C]uridine, and polyribosome fractions were then prepared from the pooled cerebral cortices of both groups, radioactive ribosomes derived from saline-treated rats were more highly aggregated than those derived from phenylalanine-treated animals. It is concluded that gross alterations in cerebral polyribosome structure and function do not occur in vivo in young rats given a large amount of phenylalanine intraperitoneally. However, this treatment, in addition to increasing ribonuclease activity in cerebral cell-free preparations, also sensitizes cerebral polyribosomes to subsequent breakdown upon exposure to ribonucleases during isolation.