Neuropil from a number of sites in both mammalian and inframammalian species were examined by electron microscopy. Complex micropinocytotic invaginations in which a tongue of cytoplasm extended into the endocytotic indentation of the cell membrane, were observed in all tissues. These complex membrane infoldings occurred at both chemical and electrotonic synapses. Coated invaginations of apposed membranes, at which infoldings occurred directly opposite each other, were also observed at the interfaces between neighboring dendrites. These observations indicate that micropinocytotic membrane infoldings involving the coordinated activity of adjacent neuronal membranes occur in a wide variety of tissues. Coordinated exocytosis/endocytosis may provide a mechanism for interneuronal bulk transport of material, both at synapses and at non-synaptic sites, in the central nervous system.