The ampullae of Lorenzini, so characteristic of sharks and rays, have been examined by electrophysiological techniques, and were found not only very sensitive to thermal stimuli, but also remarkably responsive to weak mechanical and electrical stimuli. Subsequently, Dijkgraaf and Kalmijn made a behavioral study of the electrical sensitivity of sharks and rays. They found that the shark Scyliorhinus canicula and the ray Raja clavata react reflexly to very weak electric fields in the surrounding sea water, and that ampullae of Lorenzini are the sense organs by which these reflex actions are mediated. To examine the aspect of how these sense organs respond in living animals to more natural stimuli, a method was developed by which the activity of the ampullary nerves could be recorded from live, free-swimming sharks. The preliminary results were promising, but, due to lack of experimental animals, this method could not be applied extensively in Utrecht.