1. Physiological and morphological properties of layer V non-pyramidal and pyramidal cells in isolated slices of frontal cortex from young rats (16-22 days postnatal) were studied by whole-cell, current-clamp recording of visualized cell bodies coupled with intracellular staining by biocytin at 26-27 degrees C. 2. Plotting of spike width at half amplitude against input resistance revealed two physiological categories of nonpyramidal cells. One class (n = 29) had input resistances lower than 400 M omega and spike widths at half amplitude shorter than 0.8 ms; the other (n = 22) had input resistances higher than 400 M omega and spike widths longer than 0.8 ms. According to their spike firing characteristics, the former are called fast-spiking (FS) cells, and the latter low-threshold spike (LTS) cells. 3. Resting potentials were more negative in FS cells than in LTS cells. Membrane time constants in LTS cells were four times larger than those of FS cells. Afterhyperpolarizations (AHPs) following action potentials consisted of a single component in FS cells, but two components with early and late peaks were observed in LTS cells. AHPs of FS cells had faster time-to-peak and larger amplitude than the early component of the AHPs of LTS cells. 4. Low-threshold spikes induced by depolarizing current pulses were observed at hyperpolarized potentials in LTS cells, but not in FS cells. The low-threshold spikes in LTS cells could be activated at hyperpolarized potentials by synaptic potentials. 5. Spike trains elicited by depolarizing current pulses in FS cells showed almost no spike-frequency adaptation, whereas those in LTS cells showed adaptation. 6. Excitatory postsynaptic potentials (EPSPs) of both groups of nonpyramidal cells contained N-methyl-D-aspartate (NMDA) receptor-mediated components. A combination of stimulation-induced EPSPs with depolarization caused repetitive firing in FS cells that was abolished by NMDA receptor blockers. Repetitive firing was not observed in LTS cells under these conditions. 7. The somal size of the two classes of nonpyramidal cells was similar. FS cells were all multipolar in shape, whereas LTS cells included both multipolar and bitufted types. The dendrites of some FS cells extended up into layers II/III, but there were also other FS cells with their dendrites restricted in layer V. Dendrites of LTS cells were mostly restricted to layer V. Dendrites of FS cells were mostly smooth, but those of LTS cells possessed a modest but consistent population of spines.(ABSTRACT TRUNCATED AT 400 WORDS)