The synthesis of choline phosphoglycerides and ethanolamine phosphoglycerides is noticeably decreased in brain microsomal membranes of aged rats as compared to that of two-month-old rats. Experimental evidence is presented that shows that the addition of a diglyceride preparation obtained from a polyunsaturated phosphatidylcholine to microsomes of aged rats practically restores the decreased synthesizing activity of both choline and ethanolamine phosphoglycerides. It is proposed that adding diglycerides to aged membranes affects the properties of the membranes and probably enzyme-lipid interactions.