The hydrolysis of acyl esters in phosphatidylcholine and phosphatidylethanolamine by phospholipase A in hamster heart subcellular fractions was investigated. Phosphatidylcholine was found to be a much poorer substrate than phosphatidylethanolamine for the cardiac phospholipase A. The rate of hydrolysis of phosphatidylcholine by microsomal phospholipase A was 10-fold less than with phosphatidylethanolamine as substrate. When 1-[1-14C] palmitoyl-2-acyl phosphatidyl-[Me-3H]choline was used as substrate, both phospholipase A1 and A2 activities were detected in all subcellular fractions, but the highest specific activities for both enzymes were located in the microsomal fraction. However, phospholipase A2 activity in all hamster heart particulate fractions was three to six times higher than phospholipase A1 activity. The hydrolysis of phosphatidylcholine by microsomal phospholipase A2 displayed an alkaline pH optimum and an absolute requirement for Ca2+ or Mg2+. The enzyme also depicted high specificity towards polyunsaturated acyl groups at the C-2 position of phosphatidylcholine.