Abstract— Phospholipid transfer to mqelin subfractions isolated from adult. 15-day old and 21-day old rats has been investigated by incubating myelin with radioactively labelled sonicated lecithin Lesicles and subsequently fractionating the myelin on discontinuous sucrose density gradients. The transfer process is greatly stimulated by the addition of rat brain pH 5.1 supernatant and is dependent on the amount of supernatant present in the incubation mixture and on the incubation time. Lecithin is transferred much more rapidly to the most dense mqelin subfraction than to the less dense fractions. This difference in transfer rate is most marked in myelin isolated from adult rats. The possible relevance of these obsenations to mkelin sqnthesis and turnover is discussed.