It remains to be clarified how many classes of GABAergic nonpyramidal cells exist in the cortical circuit. We have divided GABA cells in the rat frontal cortex into 3 groups, based on their firing characteristics: fast-spiking (FS) cells, late-spiking (LS) cells, and non-FS cells. Expression of calcium-binding proteins and peptides could be shown in separate groups of GABA cells in layers II/III and V of the frontal cortex: (1) parvalbumin cells, (2) somatostatin cells, (3) calretinin and/or vasoactive intestinal polypeptide (VIP) cells [partially positive for cholecystokinin (CCK)] and (4) large CCK cells (almost negative for VIP/calretinin). Combining the physiological and chemical properties of morphologically diverse nonpyramidal cells allows division into several groups, including FS basket cells containing parvalbumin, non-FS somatostatin Martinotti cells with ascending axonal arbors, and non-FS large basket cells positive for CCK. These subtypes show characteristic spatial distributions of axon collaterals and the innervation tendency of postsynaptic elements. With synchronized activity induced by cortical excitatory or inhibitory circuits, firing patterns were also found to differ. Subtype-selective occurrence of electrical coupling, finding for potassium channel Kv3.1 proteins, and cholinergic and serotonergic modulation supports our tentative classification. To clarify the functional architecture in the frontal cortex, it is important to reveal the connectional characteristics of GABA cell subtypes and determine whether they are similar to those in other cortical regions.