Rats born of mothers fed a low protein diet (8% casein) compared to control rats on a normal diet (25% casein) started 5 weeks prior to mating showed significant increases in uptake, incorporation and percent incorporation into protein of 14C-phynylalanine into brain and peripheral tissues. These effects were most pronounced on the day of birth. Also, different patterns of uptake of radioactive phenylalanine were observed between the two diet groups at birth. At ages 5, 11 and 21 days the 8% casein rats showed significant increases only in uptake and incorporation of 14C-phenylalanine into brain tissues as compared to the 25% casein animals with the percent incorporation of total radioactivity into brain protein being the same for both diet groups. For the most part, there were no significant changes in uptake and incorporation of radioactivity in peripheral tissues for the two diet groups on these post-birth days. Overall, the data indicate the possible presence of a brain specific effect whereby preferential utilization of an essential amino acid (phenylalanine) by the central nervous system occurs when rats are fed a low protein diet.