In the CNS, the exocytosis which accompanies transmitter releases occurs at the level of a presynaptic grid. Possible alterations in the grid as a function of this phenomenon were searched for at the synapses established by unmyelinated club endings on the Mauthner cell of teleosts. The number of vesicle openings generated by aldehyde fixation was diminished by cooling the preparation and enhanced after perfusion with a high-KCl Ringer solution. Morphometric analysis of the grid showed that under these conditions the mean distance between its constituent elements, the presynaptic dense projections, increased with the number of exocytotic events. Parallel changes were observed for the mean diameter of the spaces left free between these dense projections, suggesting that vesicle exocytosis produces a transient enlargement of the space where it takes place. These observations indicate that the presynaptic grid is more dynamically involved in the secretory process than previously conceived. It is therefore hypothesized that (i) the movement of the dense projections is a consequence of their interaction with the plasma membrane, and (ii) the distortion of the grid could underlie regulatory mechanisms by which the number of released vesicles is limited after each impulse. It is also proposed that the dense projections contribute to the stabilization of the plasma membrane, thereby preventing its randomization following intense release.