1.1. Rough and smooth submicrosomal fractions of normal rat liver were isolated by zonal centrifugation. The distribution and turnover of acyl moieties at the 1 and 2 positions of phosphatidylcholine and phosphatidylethanolamine, the relative rates of degradation of the major phospholipid classes, and several synthetic and hydrolytic enzyme activities associated with lipid metabolism were determined in both subfractions.2.2. We found that the fatty acid compositions of phosphatidylcholine and phosphatidylathanolamine in the submicrosomal fractions were very similar. The 1 position is predominantly occupied by saturated fatty acids and the 2 position by unsaturated fatty acids. The relative rates of degradation for the phospholipid classes in the rough microsomes are the same as those in the smooth microsomes. However, the different phospholipid species are degraded deferently within a given subfraction. Furthermore, acyl moieties esterified at the 1 position of phosphatidylcholine and phosphatidylethanolamine turned over faster than the acyl moieties esterified at the 2 position.3.3. A delicate balance of synthetic and degradative enzymes apparently accounts for the similar gross features of lipid composition and turnover in the two submicrosomal fractions derived from the endoplasmic reticulum. For example, the specific activities of 1-acylglycerol-3-phosphate acyltransferase, palmityl-CoA hydrolase, and palmityl-CoA synthetase are higher in the smooth-surfaced microsomes than in rough-surfaced microsomes. Yet, when measuring in vitro exchange (indicative of phospholipase and total acyltransferase activities) with [1-14C]oleic acid, the smooth microsomes incorporated more [14C]oleic acid into phosphatidyl-ethanolamine than the rough microsomes, but essentially no difference was observed for other lipid classes within the two submicrosomal fractions. The results obtained in vivo and in vitro are consistent with the concept that the endoplasmic reticulum is a mosaic array of structural lipoproteins and enzymic proteins in a dynamic state of heterogeneous turnover.