The constitutive enzymes of microsomal membranes were investigated during a period of rapid ER development (from 3 days before to 8 days after birth) in rat hepatocytes. The activities studied (electron transport enzymes and phosphatases) appear at different times and increase at different rates. The increase in the enzyme activities tested was inhibited by Actinomycin D and puromycin. G-6-Pase and NADPH-cytochrome c reductase activities appeared first in the rough microsomes, and subsequently in smooth microsomes, eventually reaching a uniform concentration as in adult liver. The evidence suggests that the enzymes are synthesized in the rough part, then transferred to the smooth part, of the ER. Changes in the fat supplement of the maternal diet brought about changes in the fatty acid composition of microsomal phospholipids but did not influence the enzymic pattern of the suckling. Microsomes from 8-day-old and adult rats lose 95% of PLP and 80% of NADH-cytochrome c reductase activity after acetone-H₂O (10:1) extraction. However, one-half the original activity could be regained by adding back phospholipid micelles prepared from purified phospholipid, or from lipid extracts of heart mitochondria, or of liver microsomes of 8-day or adult rats, thus demonstrating an activation of the enzyme by nonspecific phospholipid. The results suggest that during development the enzymic pattern is not influenced by the fatty acid or phospholipid composition of ER membranes.