In guinea pig heart homogenate, 34% of both choline and ethanolamine phosphoglycerides were in the form of plasmalogens (1-alkenyl, 2-acyl glycerophospholipid). Plasmalogens accounted for 39% of the choline phosphoglycerides and 36% of the ethanolamine phosphoglycerides in the mitochondrial fraction. Ethanolamine plasmalogen was the major ethanolamine phosphoglyceride (63%) in the guinea pig heart microsomal fraction. A high arachidonyl content was found in both diacyl and 1-alkenyl, 2-acyl glycerophosphoethanolamine. The C-2 fatty acyl profiles of the diacyl and 1-alkenyl, 2-acyl choline phosphoglycerides differed considerably from each other in the homogenate as well as in the subcellular fractions. Significant differences in the C-2 fatty acyl profiles also were observed in diacyl and 1-alkenyl, 2-acyl ethanolamine phosphoglycerides. Such differences suggest there is no direct metabolic relationship between the diacyl glycerophosphocholine (-ethanolamine) and its plasmalogen analog.