In in vitro slices prepared from rat sensorimotor cortex, intracellular recordings were obtained from 107 layer V pyramidal neurons, subsequently injected with biocytin for morphological reconstruction. Of the 107 neurons, 59 (55.1%) were identified as adapting (45) or non-adapting (13) regular spiking neurons (RS), and 48 (44.9%) as intrinsically bursting (IB) neurons discharging with an initial cluster of action potentials, which tended to recur rhythmically in a subset of 19 cells. The block of IAR by extracellular Cs+ did not affect burst generation, but enhanced the tendency to reburst in IB neurons. A similar effect was induced by other procedures affecting K(+)-dependent post-burst hyperpolarization. In IB neurons Ca2+ spikes had a longer decay time than in RS neurons, however selective blockers of both low and high threshold Ca2+ conductances failed to impair bursting activity. On the contrary, the perfusion of the slices with 0.5-1 microM TTX suppressed bursting behaviour in a critical time interval preceding the complete block of Na(+)-dependent action potentials. It is concluded that the persistent Na+ current INAP is the most important intrinsic factor for the typical firing properties of IB neurons, while Ca2+ and K+ conductances appear to contribute towards shaping bursts and controlling their recurrence rate. The morphology, connectivity and physiological properties of adapting and non-adapting RS neurons are particularly suited to the processing of respectively phasic and tonic inputs, whereas the properties of IB neurons are consistent with their suggested role in cortical rhythmogenesis and in the pathophysiological synchronized activities underlying epileptogenesis.