It is shown in rat that antiserum alpha 12B/18 specifically labels a new lamellar organelle that is exclusively located in dendritic appendages. These dendritic lamellar bodies occur in a restricted number of brain regions, which include areas like the inferior olive, area CA1 and CA3 of the hippocampus, dentate gyrus, olfactory bulb, and cerebral cortex. In these regions the neurons with lamellar bodies form dendrodendritic gap junctions. Immunoreactivity in the inferior olive is first detected between P9 and P15, which coincides with the development of gap junctions in this nucleus. In the adult inferior olive, the density of dendritic lamellar bodies is highest in the rostral medial accessory olive, the subnucleus where electrotonic coupling is most prominent. Antiserum alpha 12B/18, thus, specifically detects a new neuronal organelle that may be related to dendrodendritic gap junctions.