In vitro intracellular recordings were made from neurons in the rat midbrain slice. Two neuronal types could be distinguished in dopamine- containing (DA) midbrain regions based on electrophysiological criteria. One neuron type exhibited short duration action potentials (less than 1.5 msec), could fire at high frequencies (greater than 10 Hz), and exhibited either phasic or burst firing patterns. This neuron did not exhibit tyrosine hydroxylase immunoreactivity. A second neuronal type exhibited a unique set of electrophysiological properties, which included (1) a spontaneous pacemaker-like depolarizing potential, (2) a highly regular firing pattern, (3) long duration (greater than 2 msec) action potentials, and (4) a high (i.e., depolarized) spike threshold. This neuron was consistently double labeled using intracellular staining and immunocytochemical localization of the catecholamine-specific enzyme tyrosine hydroxylase, and thus represented the DA neuronal type. Midbrain DA neurons stained with Lucifer yellow could be separated into 3 classes based on their location and morphology: (1) fusiform neurons with laterally projecting dendrites in the dorsal substantia nigra zona compacta region, (2) multipolar cells with laterally and ventrally projecting dendrites in the ventral substantia nigra zona compacta, and (3) neurons with fusiform and multipolar somata and radially projecting dendrites in the ventral tegmental area. The dendrites also exhibited spine-like protrusions and ended with specialized forked processes. Spontaneously firing DA cells recorded in vitro had a number of distinguishing electrophysiological characteristics in common with those of DA neurons recorded in vivo, such as the presence of a slow depolarizing potential driving spike activity and a characteristic depolarized spike threshold (approximately-36 mV). However, in contrast to that found in vivo, the DA cells characterized here exhibited substantially higher input resistances and fired spontaneously in a very regular pacemaker pattern. Burst firing was not observed. Spike activity was apparently dependent on 4 depolarizing events: (1) a voltage-dependent TTX- sensitive slow depolarization, (2) a cobalt-sensitive low threshold depolarization that was activated during the rebound from brief membrane hyperpolarizations, (3) high threshold dendritic calcium spikes which gave rise to the spike afterhyperpolarization, and (4) a high threshold initial segment sodium spike. These depolarizations were modulated by several processes, including a 4-aminopyridine-insensitive delayed repolarization, an instantaneous and time-dependent anomalous rectifier, and an afterhyperpolarization. Although low threshold depolarizations and rebound action potentials could be triggered by the membrane repolarization following small membrane hyperpolarizations, comparatively larger hyperpolarizations attenuated this rebound activation, thereby suppressing anodal break excitation.(ABSTRACT TRUNCATED AT 400 WORDS)