1. The labelling of guinea-pig pancreas phospholipids in vivo after intraperitoneal injection of [(32)P]orthophosphate is described. 2. Acyl-CoA synthetase activity in pancreas homogenates has been studied. There is no absolute requirement for added fatty acids, indicating an adequate supply of endogenous fatty acids in these preparations. 3. Phosphatidic acid is formed in guinea-pig pancreas preparations by two distinct routes, namely the acylation of l-3-glycerophosphate and the phosphorylation of 1,2-diglyceride. Phosphatidic acid formed by either mechanism is converted into phosphatidylinositol by guinea-pig pancreas in vitro. 4. The enzymes of pancreas that convert phosphatidic acid into phosphatidylinositol via CDP-diglyceride have been characterized. 5. Addition of bovine serum albumin is necessary in assaying certain of these enzymes.