Synaptosomes (P(2)B fraction) have been isolated from the brains of 1-, 7-, 14-, 21-, 28- and 56-day-old rats. The synaptosomal concentrations of free amino acids and membrane phospholipids were analyzed for these times and expressed as synaptosomal content g(?1) brain. The content of the acidic phospholipids, phosphatidylserine and phosphatidylinositol rose sharply between days 1 and 14, remaining relatively constant thereafter. Sphingomyelin content increased through day 21. The neutral phospholipids, phosphatidylethanolamine and phosphatidylcholine, increased through day 14 and decreased thereafter. The plasmalogen forms of these phospholipids varied similarly. In keeping with the increasing density of synaptosomes in the brain over the first 3 weeks of life, synaptosomal free amino acids increased over this period. Taurine was unique inasmuch as synaptosomal content decreased somewhat between days 14 and 21. Also unique was the strong correlation between taurine concentration of the synaptosomal fraction and the phosphatidylethanolamine: phosphatidylcholine ratio.