The membrane properties and synaptic physiology of developing cat substantia nigra (SN) neurons were studied in in vitro slice preparations. Stable intracellular recordings were obtained from 46 neurons in 20 kittens ranging in age from fetal day (F) 51 to postnatal day (P) 120. Only two of these properties changed with development. The percentage of cells displaying inward rectification and the percentage of cells that generated low-threshold Ca++ spikes increased with age. Properties that did not change included resting membrane potentials, action potential amplitudes and durations, and input resistances. At all ages locally evoked synaptic responses consisted of sequences of excitatory postsynaptic potentials followed by inhibitory postsynaptic potentials. Most of the cells recorded had the electrophysiological properties which have been attributed to SN dopamine-containing neurons. To identify neurons morphologically, and verify the recording site, cells were filled with Lucifer yellow at the end of each experiment. Somatic shapes varied widely from oval to fusiform to triangular. Somatic diameters and dendritic length increased with development. Filopodial processes and growth cones were present up to the first postnatal month. Dye-coupling occurred only in the fetal group. These results indicate that cat SN neurons have many mature physiological properties during late fetal and early postnatal development. This contrasts with the significant maturation that occurs in cat caudate neurons during the same developmental period.