The nigrostriatal dopamine (DA) system is crucial to a large array of behaviors ranging from voluntary motor function to higher cognitive processes. Nigrostriatal neurons in vivo fire in three distinct patterns, modulated by afferent input. At least 70% of the inputs to nigral DA neurons are GABAergic, arising from the striatum, the globus pallidus, the rostromedial tegmental nucleus, and axon collaterals from the substantia nigra pars reticulata (SNr) projection neurons. Manipulation of these inputs, in particular those originating in SNr, produces dramatic effects on nigrostriatal neuron firing patterns. Since many or most of the afferents to the substantia nigra synapse on both nigrostriatal DA neurons and SNr GABAergic projection neurons, the response of nigral DA neurons to both excitatory and inhibitory afferents reflects a complex interplay between the intrinsic properties of the nigrostriatal neurons, the patterns and rate of the afferent input, and the synaptic interactions between SNr GABAergic neurons and the nigrostriatal DA neurons.