The most predictable anxiolytic effects of neurotransmitters are linked to the activation of a gamma-aminobutyric acid (GABA)-ergic subsystem associated with specific benzodiazepine receptors. Recent studies have indicated that subtypes of benzodiazepine receptors may be associated specifically with anxiolytic actions. Animal studies suggest that some forms of anxiety are mediated through the noradrenergic system, but a recent study testing this hypothesis confirmed it only partially. Other data implicate the serotonergic system in at least some types of anxiety. Currently the role of other neurotransmitters, such as dopamine, histamine, acetylcholine, and peptides, appears to be minimal. Clinical responses to drugs suggest that existence of at least two types of anxiety disorders representing perhaps different psychobiologic mechanisms.