1. On fractionation of guinea-pig forebrain homogenates by differential and gradient-density centrifugation most of the polyphosphoinositides were recovered in the myelin-rich particles. 2. The phospholipids of pure preparations of myelin contained di- and tri-phosphoinositide in proportions 2–3 times greater than in the whole-brain phospholipids. 3. Di- and tri-phosphoinositide appeared in young rat brain during the period of myelination. 4. After the administration of [32P]phosphate to guinea pigs the labelling of the polyphosphoinositides in isolated pure myelin was as great as in the whole brain, whereas little synthesis of the other myelin phospholipids had occurred. 5. When brain subcellular fractions were incubated with [γ-32P]ATP, some triphosphoinositide labelling occurred in the myelin-rich fraction whereas the active labelling of diphosphoinositide was localized mainly in the mitochondrial fraction. 6. The Na+, K+ and Mg2+ plus Ca2+ concentrations in purified myelin have been determined. The Mg2+ plus Ca2+ content present showed close acid–base equivalence to the polyphosphoinositides. 7. It is concluded that di- and tri-phosphoinositide are rapidly-metabolizing components of the myelin sheath or intimately associated structures.