For decades, scientists have believed that anxiety results from abnormalities in brain chemistry. They based this conviction on the effects of drugs that reduce anxiety by increasing the availability of certain neurotransmitters in the brain. The first antianxiety drugs were benzodiazepines, which raise levels of the neurotransmitter gamma-aminobutyric acid (GABA). Later, drugs that increase serotonin levels and affect norepinephrine and other neurotransmitters associated with mood also proved effective. But these findings have raised even more questions. For example, what brain structures are involved? What malfunctions in the brain induce anxiety? And what role do neurotransmitters play?