Effects of phospholipase A2-activators, melittin and mastoparan, on rat anterior pituitary cells were studied by use of the electron microscope. Rat anterior pituitaries were incubated in HEPES buffer containing 20 micrograms/ml of melittin or the same dose of mastoparan for 5 min, 10 min and 20 min. Features indicating discharge of granule contents by exocytosis were increased with time, and the simultaneous extrusion of a number of secretory granules, named "multigranular exocytosis", was often recognized in addition to single-granule exocytosis at 10 min and 20 min. Most membrane pits, where the multigranular exocytosis as well as the single-granule exocytosis occurred, were coated. Moreover, a large number of vesicles coated or non-coated were distributed near the trans side of the Golgi apparatus of melittin-treated or mastoparan-treated cells after 20 min. These vesicles might be related to membrane internalized from the excess surface membrane derived from the limiting membrane of exocytosed granules. These observations indicate that phospholipase A2-activators induce hormone release involving membrane fusion between limiting membranes of secretory granules, and between granule-limiting membrane and plasma membrane in rat anterior pituitary cells.