1. The acridine derivative quinacrine was used to stain nerve fibres in internal organs of 17 species of marine and terrestrial invertebrates from seven different phyla. 2. The majority of preparations showed quinacrine-stained nerve fibres. There was a degree of variation, ranging from a dense network of nerve bundles to single fibres. Quinacrine-positive nerve cell bodies were also observed in some ganglia. 3. Pharmacological experiments were performed on a variety of isolated tissues from the different marine and terrestrial groups, in order to ascertain their sensitivity to adenine nucleosides and nucleotides. 4. Correlation is drawn between the ability of neurones within a tissue to bind quinacrine, and the ability of that tissue to respond to applied adenine nucleosides and nucleotides.