The present experiments were conducted to determine the electrophysiological and pharmacological properties of substantia nigra neurons in the mouse. These cells were studied using extracellular single unit recording and microiontophoretic techniques in both chloral hydrate anesthetized mice and in vitro mouse slices. In the in vivo preparation the substantia nigra zona compacta neurons had long duration action potentials (greater than 4 ms), fired from 1 to 7 impulses/s, and the cells discharged with either a decremental burst pattern or with a regular pattern. The dopamine agonists apomorphine and d-amphetamine, given systemically, decreased the firing rate of these neurons and the dopamine receptor blocker, haloperidol, reversed these effects. The zona compacta neurons were inhibited by the micro-iontophoretic application of dopamine and gamma-aminobutyric acid, and systemic haloperidol selectively attenuated the effects of dopamine. In vitro recordings from substantia nigra zona compacta and zona reticulata neurons were generally similar to those found in vivo, both in terms of the electrophysiological and pharmacological properties. However, the zona compacta cells fired faster in vitro than in vivo, and the firing pattern in vitro tended to be pacemaker-like, especially when recordings were made in an incubation medium which blocks synaptic transmission (e.g. low Ca2+/high Mg2+). Our data indicate that: (a) in vivo mouse zona compacta neurons exhibit the same electrophysiological and pharmacological properties as rat dopamine-containing neurons; (b) in vitro mouse dopaminergic neurons fire with pacemaker regularity when in a low Ca2+/high Mg2+ environment; and (c) in vitro studies offer an approach to examine the basic properties of dopaminergic neurons exclusive of feedback pathways and other afferent inputs.