Biochemical investigation of receptors for neurotransmitters and drugs in the brain has been one of the most active areas of molecular neuroscience during the past decade. This work has permitted fundamental insights into how binding of neurotransmitters to their receptors excites or inhibits neuronal firing or changes cellular metabolism. The recognition of receptor subtypes has suggested subtle ways for neurotransmitters to modulate neuronal functioning. Finally, the ability to measure receptor sites in simple test tube systems and to distinguish readily between agonists and antagonists has provided useful probes for drug discovery programs.