To further define the operational boundaries on fast inhibition in neocortex, whole cell recordings were made from layer V pyramidal neurons in neocortical slices to evaluate evoked inhibitory postsynaptic currents (IPSCs) and spontaneous miniature IPSCs (mIPSCs). Stimulating electrodes were placed in layers VI and I/II to determine whether simultaneous stimulation of deep and superficial laminae could extend the magnitude of maximal IPSCs evoked by deep-layer stimulation alone. The addition of superficial-layer stimulation did not increase maximal IPSC amplitude, confirming the strict limit on fast inhibition. Spontaneous miniature IPSCs were recorded in the presence of tetrodotoxin. The frequency of spontaneous mIPSCs ranged from 10.0 to 33.1 Hz. mIPSC amplitude varied considerably, with a range of 5. 0-128.2 pA and a mean value of 20.7+/-4.1 pA (n = 12 cells). The decay phase of miniature IPSCs was best fit by a single exponential, similar to evoked IPSCs. The mean time constant of decay was 6.4+/-0.6 ms, with a range of 0.2-20.1 ms. The mean 10-90% rise time was 1.9+/-0.2 ms, ranging from 0.2 to 6.3 ms. Evaluation of mIPSC kinetics revealed no evidence of dendritic filtering. Amplitude histograms of mIPSCs exhibited skewed distributions with several discernable peaks that, when fit with Gaussian curves, appeared to be spaced equidistantly, suggesting that mIPSC amplitudes varied quantally. The mean separation of Gaussian peaks ranged from 6.1 to 7.8 pA. The quantal distributions did not appear to be artifacts of noise. Exposure to saline containing low Ca(2+) and high Mg(2+) concentrations reduced the number of histogram peaks, but did not affect the quantal size. Mean mIPSC amplitude and quantal size varied with cell holding potential in a near-linear manner. Statistical evaluation of amplitude histograms verified the multimodality of mIPSC amplitude distributions and corroborated the equidistant spacing of peaks. Comparison of mIPSC values with published data from single GABA channel recordings suggests that the mean mIPSC conductance corresponds to the activation of 10-20 GABA(A) receptor channels, and that the release of a single inhibitory quantum opens 3-6 channels. Further comparison of mIPSCs with evoked inhibitory events suggests that a single interneuron may form, on average, 4-12 functional synapses with a pyramidal cell, and that 10-12 individual interneurons are engaged during recruitment of maximal population IPSCs. This suggests that inhibitory circuits are much more restricted in both the size of the unit events and effective number of connections when compared with excitatory inputs.