1. Large aspiny neurons (20-60 microns diam) in the neostriatum were studied in an in vitro rat slice preparation by whole-cell recording to reveal physiological identification from medium-sized spiny projection cells (10-20 microns diam), relation to the patch and matrix compartments, and excitatory synaptic inputs. Recorded cells were identified by intracellular biocytin staining. Compartmental identification was made by calbindinD28K immunohistochemistry in fixed slices. 2. Large stained neurons were morphologically heterogeneous and had aspiny or sparsely spiny dendrites and dense local axonal branches. They were located in the matrix or on the patch-matrix border. Axonal branches of the large aspiny cells were preferentially distributed in the matrix and gave off terminal boutons there. Some of the secondary dendrites arising from stem dendrites running along the border, however, crossed compartment boundaries and made fine branches in a patch. 3. Large aspiny cells had less negative resting membrane potentials and lower thresholds for spike generation than medium spiny cells. They showed longer-duration and larger-amplitude afterhyperpolarizations (AHPs) than medium spiny cells. During hyperpolarizing current pulses, apparent resistance slowly reduced, and a prominent sag was observed in the voltage record, which was absent in medium spiny cells. The large aspiny cells showed no spontaneous firing but had a tendency to fire repetitive spikes in response to depolarizing current pulses, although spike interval tended to increase in later spikes. Spike frequency of large aspiny cells increased less with current intensity than that of medium spiny cells. 4. Most large aspiny cells were considered to belong to a single physiological class, although one large aspiny cells showed shorter-duration AHPs than both most other large aspiny cells and medium spiny cells, and little spike-frequency adaptation. 5. Excitatory postsynaptic currents (EPSCs) of large aspiny cells induced by intrastriatal stimulation had two components. An early, linear component was blocked by 10 microM 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX), a selective antagonist of non-N-methyl-D-aspartate (NMDA) receptors. A later component with a nonlinear current-voltage (I-V) relationship was blocked by 50 microM DL-2-amino-5-phosphonovaleric acid (DL-APV), a selective antagonist of NMDA receptors. 6. From these results, four conclusions can be drawn. 1) Most large aspiny neostriatal cells in the matrix, although they take heterogeneous shapes, belong to one physiological class with long-duration AHPs and a strong time-dependent component of anomalous rectification.(ABSTRACT TRUNCATED AT 400 WORDS)