Publisher Summary This chapter describes the pharmacology of thalamic and geniculate neurons. It discusses the progress that has been made in identifying transmitter agents in brain. The substances discussed are acetylcholine (ACh), monoamines, including dopamine, noradrenaline, and 5-hydroxytryptamine, and the neutral and acidic amino acids, such as γ-aminobutyric acid (GABA) and glutamic acid. The problem of transmitter identification should be approached with caution. The complex organization of the central nervous system with a close proximity of many neurons, glial cells, and capillaries makes it difficult to study the actions of putative transmitters on single neurons in isolation. The complexity of the synaptic junction itself is such that several possible mechanisms of drug action must be considered before the actions of a particular substance upon a neuron can be evaluated. A series of criteria have been developed from the classical studies on the peripheral nervous system, which has established acetylcholine as a transmitter at neuromuscular and ganglionic synapses. These criteria constitute a useful guide in any assessment of the possibility that a substance is a transmitter at a particular synaptic junction.