In the first two reports of this series, in vivo intracellular recording techniques were used to characterize the electrophysiological properties of two types of striatal neurons that had been identified by their distinct response patterns to stimulation of corticostriatal afferents. In this paper, we examined whether cells showing Type I or Type II response patterns also differed with respect to their morphology or compartmental localization by combining intracellular recording and Lucifer yellow staining with immunocytochemical localization of calbindin 28 kd immunoreactivity. In the majority of cases, both Type I and Type II neurons exhibited similar morphological characteristics, with 80% of the Type I cells (13/16) and all of the Type II cells (n = 40) being small or medium spiny neurons. In each case where the morphological class of the cell was different than the spiny cell class, the cell exhibited a Type I response pattern. These spiny neurons had somata that averaged 17.1 +/- 1.3 microns in diameter and gave rise to between four and eight primary dendrites. The axons typically arose from cell bodies (7/13 for Type I and 25/40 for Type II cells) and emitted extensive local axonal collaterals. However, the axons of Type I cells more frequently originated from the dorsal surface of the somata (9/13; 69%), whereas Type II axons more frequently arose from the ventral surface of the somata (25/35; 71%), which may account for their different extracellular waveforms. In coronally sectioned tissue (n = 18), the axons always projected laterally when the somata were located in the medial striatum and projected medially when the somata were in the lateral striatal region. In a subset of experiments (N = 22), Lucifer yellow-stained neurons were localized with respect to their position within the patch and matrix compartments of the striatum using subsequent staining for calbindin 28 kd immunoreactivity. Of the 20 labeled medium spiny neurons examined (Type II: N = 13; Type I: N = 7), 19 were located in the calbindin-positive matrix compartment. The only neuron localized to the patch compartment was a medium spiny cell that exhibited a Type II paired impulse response pattern. In addition, of the two aspiny neurons from this group with beaded dendrites, one was localized to the border between adjacent patch and matrix compartments, whereas the other was located completely within the matrix compartment. Therefore, despite their distinct paired impulse response patterns, the majority of both Type I and Type II neurons were medium spiny cells located in the matrix compartment of the striatum.(ABSTRACT TRUNCATED AT 400 WORDS)