Abstract. The lipophilic fluorescent dye, FM1-43, as now frequently used to stain cell membranes and to monitor exo-endocytosis and membrane recycling, induces a cortical [Ca2+]i transient and exocytosis of dense core vesicles (``trichocysts) in Paramecium cells, when applied at usual concentrations (10 m) in presence of extracellular Ca2+ ([Ca2+]o= 50 m). When [Ca2+]o is kept at 30 nm ([Ca2+]resti), in about one third of the population of extrudable trichocysts docked at the cell membrane, FM1-43 induces membrane fusion, visible by FM1-43 fluorescence of the vesicle membrane. However, in this system extrusion of secretory contents cannot occur in absence of any sufficient Ca2+o. Upon readdition of Ca2+o or some other appropriate Me2+o at 90 m, secretory contents can be released (complete exocytosis). Resulting ghosts formed in presence of Ca2+, Sr2+ or Mn2+ are vesicular, but when formed in presence of Mg2+, for reasons to be elucidated, they are tubular, though both types are endocytosed and lose their FM1-43 stain. In contrast, in presence of [Mg2+]o= 3 mm (which inhibits contents release), the exocytotic openings reseal and intact trichocysts with labeled membranes and with still condensed contents are detached from the cell surface (``frustrated exocytosis) within 15 min. They undergo cytoplasmic streaming and saltatory redocking, with a half-time of 35 min. During this time, the population of redocked trichocysts amenable to exocytosis upon a second stimulus increases with a half-time of 35 min. Therefore, acquirement of competence for exocytotic membrane fusion may occur with only a small delay after docking, and this maturation process may last only a short time. A similar number of trichocysts can be detached by merely increasing [Mg2+]o to 3 mm, or by application of the anti-calmodulin drug, R21547 (calmidazolium). Essentially we show (i) requirement of calmodulin and appropriate [Me2+] to maintain docking sites in a functional state, (ii) requirement of Ca2+o or of some other Me2+o to drive membrane resealing during exo-endocytosis, (iii) requirement of an ``empty signal to go to the regular endocytotic pathway (with fading fluorescence), and (iv) occurrence of a ``filled signal for trichocysts to undergo detachment and redocking (with fluorescence) after ``frustrated exocytosis.