The benzodiazepines are among the most widely used drugs in the world. When first introduced, little was known about their mechanism of action. However, in the last 20 years, our understanding of the chemistry and function of the central nervous system (CNS) has increased substantially. This knowledge has shed some light on the mechanism of action of the benzodiazepines and other centrally acting drugs. It is well established that the benzodiazepines act by combining with specific receptors in the central nervous system. These receptors are anatomically in close association with gamma amino butyric acid (GABA) receptors and appear to reside on the neuronal membrane in the same supramolecular protein complex. GABA is the major inhibitory neurotransmitter of the CNS. The benzodiazepines act by increasing the affinity of the GABA receptor for its ligand, thereby augmenting the inhibitory effect of a given concentration of GABA. Two hypotheses of benzodiazepine ligand-receptor interactions in this supramolecular protein complex have been proposed: (1) multiple receptor subtypes analogous to the opioid receptors; (2) single receptor with multiple conformations. The multiple receptor hypothesis suggests that each pharmacologic effect of the benzodiazepines (i.e., anxiolysis) is mediated by interaction with a specific receptor subtype. On the other hand, the alternative hypothesis suggests that only one receptor exists which has a dynamic conformation. Experimental evidence in support of each hypothesis is presented and critically evaluated.