Gap junctions are ubiquitous channels that serve a vital function in communication between cells. They control growth and development and coordinate diverse functions within multicellular organisms. In the nervous system, gap junctions transmit electrical signals rapidly between cells. The protein subunit of the gap junction is known as the connexin in vertebrates and the innexin in invertebrates. Drosophila has eight innexin genes and is used as a model organism in this study of gap junctions. Structural details of the gap junctions were examined using polyclonal antibodies raised against the carboxy-terminal tail of two innexin proteins, Shaking-B and Innexin2. Immunoelectron microscopy, in combination with low temperature embedding, was used to label Shaking-B gap junctions in the region of the Giant-Fibre synapses. The labelling was precisely located on the intracellular face, confirming the predicted topology of this protein. Immunoelectron microscopy using Inx2 antibody labelled gap junctions throughout the embryonic epidermis and larval wing discs. The inx2 gene is expressed in the central nervous system, epidermis, foregut, hindgut, trachea and imaginal discs. Some inx2 mutants are reported to have defects in the proventriculus which prevent feeding. The feeding behaviour of inx2 semi-lethal mutants was examined. These mutants ingest only small quantities of food, although the proventriculi of these, and of null mutants, appears morphologically normal. In addition, inx2 null mutants were not rescued from embryonic lethality when inx2 was expressed in the mutant background by armadillo and inx2 promoters using the UAS- GAL4 enhancer trap system. inx2 appears to be a crucial gene in Drosophila development, but its site and time of function remain to be determined. Expression of innexin genes in the moth, Manduca sexta and Dipteran fly Lucilia were examined in brain sections by cross hybridisation with Drosophila inx2, inx3, inx4 and inx7 probes. All probes appear to hybridise to the cellular cortex. This pattern of expression differs from that of Drosophila.