Although it had been suggested that chemical transmission was an important physiological mechanism, confirmation awaited Loewi's finding that autonomic nerve impulses resulted in the release of specific chemical stimulants. These results led to an inevitable conclusion--that there must be specific recognition sites that can respond to the presence of the minute amounts of these chemical transmitters. In the intervening years, it became clear that the interaction of a ligand (e.g., a chemical transmitter) with a binding site (e.g., a receptor) could be characterized not only physiologically, but also biochemically. However, the available biochemical methods required relatively large amounts of tissue containing the receptors, especially considering the low concentration of receptors in the nervous system.