After injection, labelled stearic acid is transported directly into the brain and incorporated into brain lipids without prior oxydation to acetate and resynthesis of fatty acids. Contamination by blood can be excluded. (The preparation contains all subcellular fraction except cytosol). The labelled stearic acid taken up is partly metabolized in the brain either by elongation or by degradation and in situ resynthesis of fatty acids. The activity in oleic acid and mono-unsaturated chains is hardly detectable. The labelled acids are incorporated into lipids or subcellular particles following characteristic kinetics, which show a diminution by 24 hours. When analysing the evolution of each lipid, it is shown that this profile is followed by phospholipids, but not by cerebrosides and free fatty acids. The formers are still increasing up to 50 hours, the latters are stable (suggesting a physical binding between membranes and free fatty acids). Thus nutrition is an important parameter for the synthesis of brain membranes as far as exogenous saturated fatty acids are needed.