Stimulation of secretion with A23187 circumvents the usual mechanism of stimulation of secretion by direct mediation of an increase in cytoplasmic Ca++ and thereby permits study of the terminal components of the secretory process in which granules are externalized by membrane fusion events. Two alterations in the plasma membrane precede fusion: the formation of bulges and the aggregation of intramembranous particles. These changes require a permissive level of ATP and are sensitive to reagents that bind to intracellular protein sulfhydryl groups. They seem not to be attributable to a direct effect of Ca++ on membrane phospholipids. The cell components responsive to Ca++ and responsible for the alterations in the membrane are not known; neither microtubules nor actin filaments seem to qualify.