1. A tissue slice containing a section of striatum was cut obliquely from rat brain so as to preserve adjacent cortex and pallidum. Intracellular recordings were made from 368 neurones, using either conventional or tight-seal configurations. 2. Two types of neurone were distinguished electrophysiologically. Principal cells (96%) had very negative resting potentials (-89 mV) and a low input resistance at the resting membrane potential (39 M omega): membrane conductance (10 nS at -65 mV) increased within tens of milliseconds after the onset of hyperpolarization (99 nS at -120 mV). Secondary cells (4%) had less negative resting potentials (-60 mV) and a higher input resistance (117 m omega at the resting potential): hyperpolarization caused an inward current to develop over hundreds of milliseconds that had the properties of H-current. 3. Most principal cells were activated antidromically by electrical stimulation of the globus pallidus or internal capsule. Intracellular labelling with biocytin showed that principal cells had a medium sized soma (10-18 microns), extensive dendritic trees densely studded with spines and, in some cases, a main axon which extended towards the globus pallidus. 4. Electrical stimulation of the corpus callosum or external capsule evoked a depolarizing postsynaptic potential. This synaptic potential was reversibly blocked by a combination of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and DL-2-amino-5-phosphonovaleric acid (APV, 30 microM), but was unaffected by bicuculline (30 microM) and picrotoxin (100 microM). The underlying synaptic current had a fast component (time to peak about 4 ms), the amplitude of which was linearly related to membrane potential and which was blocked by CNQX; in CNQX the synaptic current had a slower component (time to peak about 10 ms) which showed voltage dependence typical of N-methyl-D-aspartate (NMDA) receptors. Both currents reversed at -5 mV. 5. Focal electrical stimulation within the striatum (100-300 microns from the site of intracellular recording) evoked a synaptic potential that was partially blocked (45-95%) by CNQX and APV: the remaining synaptic potential was blocked by bicuculline (30 microM). The bicuculline-sensitive synaptic current reversed at the chloride equilibrium potential. 6. The findings confirm that the majority of neostriatal neurones (principal cells, medium spiny neurones) project to the pallidum and receive synaptic inputs from cerebral cortex mediated by an excitatory amino acid acting through NMDA and non-NMDA receptors. These cells also receive synaptic inputs from intrinsic striatal neurones mediated by GABA.(ABSTRACT TRUNCATED AT 400 WORDS)