Physiological and morphological properties of nonpyramidal cells in layer II/III of frontal cortex of young rats were studied in vitro by whole-cell recording and intracellular staining with biocytin. Layer II/III nonpyramidal cells could be divided into four subgroups by their firing patterns in response to depolarizing current pulses and their patterns of dendritic and axonal arborizations. (1) Fast-spiking (FS) cells (35% of the total sample) showed an abrupt episode of nonadapting repetitive discharges of shorter-duration action potentials. FS cells had local and horizontal axonal arbors which did not enter layer I. This type of FS cell was immunoreactive for parvalbumin and included some basket cells. Three chandelier cells were identified as FS cells, although one chandelier cell was not identified as an FS cell. (2) Late- spiking (LS) cells (11%) exhibited slowly developing ramp depolarizations near threshold. LS cells were neurogliaform cells. (3) Low-threshold spike (LTS) cells (5%) had prominent low-threshold spikes when stimulated from hyperpolarizations. The main axons of LTS cells ascended, and the collaterals entered into layer I. (4) The remaining cells [regular-spiking nonpyramidal (RSNP) cells] (49%) could not be categorized into the above three subgroups. In some RSNP cells depolarizing pulses from hyperpolarizations induced fast depolarizing notches with small amplitude. RSNP cells had vertically elongated axonal fields, extending from layer I to V, sometimes to layer VI. This subgroup included double bouquet cells and bipolar cells. Each subgroup with a different firing mode may differentially contribute to neocortical laminar and columnar circuitry.