The rate of turnover of individual P-compounds, related to the metabolic cycle of some phosphatides, was followedin vivo in the brain of rats a few days old after³²P administration. The phosphorylcholine is the precursor of the cytidinediphosphatecholine; this in turn may be the precursor of lecithin. Thel--glyceryl-phosphorylcholine represents the degredation product of lecithin. The specific activity curves of the phosphorylethanolamine and phosphorylse rine are similar to that of the phosphorylcholine; likewise thel--glycerylphos-phorylethanolamine is similar to the activity curve of thel--glycerylphosphorylcholine.