Enzymes associated with glycerolipid biosynthesis were examined in microsomal fractions of liver and adipose tissue obtained from swine of various ages. Generally, liver glycerophosphate acyltransferase, phosphatidate phosphohydrolase, diglyceride acyltransferase, and choline phosphotransferase activities were substantial at birth but increased 2- to 3-fold by day 14 postpartum, decreased at day 25, then increased at the oldest ages studied (up to 155 days postpartum). In adipose tissue, enzyme activities were low at birth and developed through day 25 in a pattern generally similar to that observed in liver. In contrast to liver, the adipose enzymes were depressed immediately postweaning (day 32) with subsequent recovery. The observed decline in adipose tissue enzyme activities expressed on a tissue basis at older ages was primarily the result of increased adipocyte size, since the activities expressed on a cell basis did not decline as rapidly. In both liver and adipose tissue, phosphatidate was the major glycerolipid synthesized by the microsomal glycerophosphate acyltransferase enzymes at all ages (generally greater than 75%). The ratio of neutral lipids to phospholipids produced by acylation of glycerophosphate was increased when a microsomal--cytosolic preparation was used as a source of enzyme in contrast to a microsomal preparation.