Mixed-rod cone bipolar (Mb) cells of goldfish retina have large synaptic terminals (10 microm in diameter) that make 60-90 ribbon synapses mostly onto amacrine cells and rarely onto ganglion cells and, in return, receive 300-400 synapses from gamma-aminobutyric acid (GABA)-ergic amacrine cells. Tissue viewed by electron microscopy revealed the presence of double-membrane-bound processes deep within Mb terminals. No membrane specializations were apparent on these invaginating processes, although rare vesicular fusion was observed. These invaginating dendrites were termed "InDents". Mb bipolar cells were identified by their immunoreactivity for protein kinase C. Double-label immunofluorescence with other cell-type-specific labels eliminated Müller cells, efferent fibers, other Mb bipolar cells, dopaminergic interplexiform cells, and somatostatin amacrine cells as a source of the InDents. Confocal analysis of double-labeled tissue clearly showed dendrites of GABA amacrine cells, backfilled ganglion cells, and dendrites containing PanNa immunoreactivity extending into and passing through Mb terminals. Nearly all Mb terminals showed evidence for the presence of InDents, indicating their common presence in goldfish retina. No PanNa immunoreactivity was found on GABA or ganglion cell InDents, suggesting that a subtype of glycine amacrine cell contained voltage-gated Na channels. Thus, potassium and calcium voltage-gated channels might be present on the InDents and on the Mb terminal membrane opposed to the InDents. In addition to synaptic signaling at ribbon and conventional synapses, Mb bipolar cells may exchange information with InDents by an alternative signaling mechanism.