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The neuroendocrine Caudo-Dorsal Cells (CDC) of the snail release an egg-laying hormone (CDCH) from their axon terminals by exocytosis. The CDC show 3 states of electrical activity, resting (inactive), active (electrical discharge) and inhibited (inexcitable). Exocytosis was first studied quantitatively with the tannic acid-glutaraldebyde-OO-method (TAGO method). Exocytosis activity appeared to be low in the resting state, very high in the active state, and moderate in the inhibited state. Active CDC particularly show multigranular exocytosis. Furthermore, with the freeze- fracture technique two stages of the exocytotic fusion process were identified: 1. the pre-exocytotic stage, characterized by particle-free areas in the P- and E-faces of the secretory granule and the opposing plasma membrane, and 2. the fusion step, visible as characteristic circular membrane discontinulties. Morphometry of freeze-fractured CDC confirmed that exocytoses are scarce during electrical rest, numerous during activity, and moderate during inhibition. The differences between the numbers of exocytoses In the 3 states were not that high as In the TAGO-studies, apparently because the freeze-fracture preparations did not allow quantitation of multigranular exocytosis. It is concluded that the TAGO-metbod appears very suitable for quantitation of exocytosis activity, whereas the freeze-fracture technique is particularly useful to study membrane events during exocytosis. The significance of exocytoals in the 3 CDC states for the control of egg-laying has been discussed. [ABSTRACT FROM PUBLISHER]