Glucagon stimulated the incorporation of Na2H32PO4 and L-(14C)serine into phosphatidylserine in heart muscle slices. The increase above control was about 2-fold at ten minutes and 6-fold at thirty minutes for (32P) and 12-fold as early as three minutes for (14C)serine. Although a smaller, but significant, incorporation of (32P) into phosphatidylethanolamine was also observed, glucagon did not stimulate the incorporation of (14C)serine into phosphatidylethanolamine. Glucagon did not significantly augment the incorporation of either tracer into phosphatidylcholine, lysophosphatidylcholine, phosphatidylinositol, cardiolipin, phosphatidic acid, or sphingomyelin. Dibutyryl cyclic 3',5'-AMP did not increase the incorporation of (32P) or (14C)serine into phosphatidylserine. Since phosphatidylserine appears to serve a critical role in coupling the glucagon receptor to the catalytic moiety of adenylate cyclase, the data suggest that the hormone may initially increase the amount of its own coupler.