In the present investigation we have shown that cytoplasmic membranes of adipocytes contain A1 and A2 phospholipase activities which are optimal in a buffer with 5 mM Ca2+ (pH 8.5). Insulin enhances these activities within phosphatidylethanolamine. Insulin increases also the amount of free fatty acids in membranes. Rodbell and Blecher have already shown an insulin-like action of phospholipases towards the uptake of glucose and amino-acids by adipocytes. Shier and Asakawa have recently described that lysolecithin and unsaturated fatty acids can change nucleotide-cyclase activities of cytoplasmic membranes towards GTP and ATP; lysolecithins and Triton X 100 seem to react in an identical way. Results from these studies give new suggestions on insulin action; phospholipase activation changes membrane physiochemical properties inducing an increase of glucose carrier mobility and leading the membrane cyclase enzyme (s) towards GMP cyclic synthesis.