When enzyme secretion was stimulated with acetylcholine about two-thirds of the increment in radioactivity in the total phospholipides was found in deoxycholate soluble components of the microsome fraction. The remainder of the increment was distributed in the other fractions. This indicates that the cellular component in which the increase in phospholipide turnover occurs on stimulation of secretion is a membranous structure. Evidence is presented which indicates that the increment in radioactivity in the non-microsomal fractions on stimulation of secretion is due to contamination of these fractions with fragments of the stimulated membranous structure.

The distribution of P³² radioactivity in each of the chromatographically separated phospholipides in the various fractions from unstimulated tissue paralleled the distribution of radioactivity in the total phospholipide fraction, indicating that individual phospholipides are not concentrated in different fractions but are associated together in the membranous structures of the microsome fraction. The major proportion of the stimulation of the turnover of the individual phospholipides also occurred in the microsome fraction.

The distribution of radioactivity from glycerol-1-C¹⁴ in the total phospholipides and in the individual phospholipides in the various fractions was similar to the distribution of P³². In the microsome fraction acetylcholine stimulated the incorporation of glycerol-1-C¹⁴ in each phospholipide which showed a stimulation of P³² incorporation.

The significance of the turnover of phosphatides in microsomal membranes in relation to the mechanism of secretion is discussed.