In the dendritic region of the supraoptic nucleus of young adult rats we found ultrastructural evidence of rapid (<30 min) morphological reorganization following transcardial perfusion of the anesthetized animals with high (325 mOsm), but not with normal, osmolality (302 mOsm) balanced salt solutions. The changes observed included significant increases in dendritic bundling and increased numbers of multiple axodendritic synapses (where one presynaptic terminal contacted two or more postsynaptic dendrites). In our initial experiments the perfusion medium contained tannic acid, but when the tannic acid was omitted in a separate set of experiments, the results were both similar and more striking. These studies indicate that the plasticity previously seen in this region under physiological conditions can occur rapidly and probably involves astrocytic process withdrawal from between adjacent dendrites. This change in the distribution of astrocytic processes allows previously single synapses access to additional postsynaptic dendritic membrane. Together, increased dendritic bundling and multiple synapses may facilitate the synchronization of cell firing and, thus, hormone release. © 1993 by Academic Press, Inc.