The study of dopamine (DA) neuron activity using electrophysiological recording techniques has engendered significant contributions in advancing our understanding of the physiology of the nigrostriatal and mesolimbic dopaminergic systems. This has been particularly evident in studies related to the neuronal effects of therapeutic drug actions. Most of these studies have used extracellular recording techniques to ascertain the involvement of DA neurons in a variety of central drug effects, such as the amphetamine-induced inhibition of DA neuron firing via activation of striatonigral feedback loops,'the differential effects of acute and chronic neuroleptic administration on DA cell activity,* and the involvement of autoreceptors in the complementary behavioral effects produced by low versus high doses of the DA agonist apomorphine.'However, although extracellular recording proved useful in assessing how neuronal firing rates are affected by drugs and stimuli, this technique is nevertheless comparatively limited in its ability to address the actual mechanisms underlying these phenomena. In contrast, the application of intracellular recording techniques to the in vivo preparation enabled the investigation of DA system activity at this more basic level of analysis. Thus, the DA cell responses previously characterized during extracellular recording could be reexamined at the level of membrane voltage and conductance changes. In vivo intracellular recordings also allowed a direct correlation to be made between specific neuronal electrophysiological characteristics and neurochemically identified DA cells. This electrophysiological identification thus enabled DA cell identification in subsequent experiments to be based on this defined set of unique electrophysiological criteria. Moreover, initial studies on DA neuron action potential generation using intracellular recording techniques provided basic information on DA cell regulation that would have been difficult if not impossible to infer based solely on the information provided by extracellular recordings. This included the driving of spontaneous spike generation by an endogenous pacemakerlike slow depolarizing current, and the calciummediated transition of firing patterns from single spiking to burst firing. 4. s Despite the higher information content of data afforded by intracellular recordings in the intact preparation, the in vivo preparation nonetheless is constrained by the inability to conclusively determine the ionic mechanisms underlying these observa-