The innervated face of electrocytes in the main electric organ of Electrophorus electricus L. was examined by light microscopy, both conventional and with Nomarski contrast, and by transmission and scanning electron microscopy. Acetylcholinesterase cytochemistry was used in the demonstration of the greater density of synapses over the caudal papillae. The various techniques contributed to a better understanding of the distribution and form of papillae and synapses at the posterior face of the electrocyte. Caudal papillae are longer and thinner than those at the rostral face, but it was not possible to recognize a different type sometimes referred to in the literature as small papillae. The contact of nerve endings with the electrocyte seems to be made predominantly on the terminal half of caudal papillae, however a smaller number occur elsewhere on the posterior face. Synaptic terminals frequently appear as round profiles, but may be also elongated, with or without bulges, usually occupying a depression, and separated from the post-synaptic membrane by a space of 60-100 nm, where an expansion may be found.