The quaternary lidocaine derivative QX-314 was applied internally to CA1 pyramidal neurons of the guinea pig hippocampal slice preparation. This local anesthetic blocked both fast Na+-dependent action potentials and the voltage-dependent, non-inactivating Na+ conductance. Partially blocked Na+ spikes exhibited pronounced frequency-dependent depression at rates as low as 0.2 Hz. Ca2+-dependent electrogenesis, synaptic potentials and glutamate-induced depolarizations were apparently unaffected even after large doses of QX-314. Evidently, the cellular mechanisms of local anesthetics on central neuronal membranes are similar to those described for peripheral axons. The frequency-dependence of spike blockade may account for some of the effects of local anesthetics on the CNS in vivo. The localized intracellular application of QX-314 may prove useful as a specific pharmacological tool in studies of central neurons.