Three phenylalanine injections (1 mg/g body wt) at 4-hour intervals resulted in increases in phenylalanine and tyrosine levels which were age- (4–42 days) and tissue (plasma, brain, liver)-dependent. Incorporation of [1-14C]-acetate into lipid was constantly reduced by 20–40% in brain but not liver of 7-, 11-, 14- and 21-day-old phenylalanine-injected rats, while in adult brain incorporation was near normal. A similar distribution of label between lipid classes of differing brain regions in injected rats, compared to controls at all ages, indicates a lack of a selective effect on myelin lipid synthesis, suggesting that reduced myelination in hyperphenylalaninemia is not related to high phenylalanine levels per se.