Action potentials, postsynaptic potentials (PSPs) and burst potentials have been recorded intracellularly from over 200 neurons in hippocampal cell cultures prepared from fetal mice of 13-18 days gestational age. Repetitively firing action potentials are elicited by intracellular electrical stimulation and often are preceded by stereotyped prepotentials which probably are generated on processes remote from the cell body. In some cells, action potentials are succeeded by long duration depolarizing afterpotentials (0.3--2 sec) with additional superimposed action potentials. Postburst afterhyperpolarization can last up to 2.5 sec. Action potentials are short (0.6--1.2 msec) with peak rates of rise from 64 to 267 V/sec (mean 139 +/- 13 V/sec, 24 cells) and corresponding rates of fall from 21 to 133 V/sec (mean 70 +/- 7 V/sec, 24 cells). Following single action potentials, the afterhyperpolarization is usually less than 10 msec. Inhibitory PSPs occur frequently (up to 70% incidence), have reversal potentials of--30 to--40 mV, and can be evoked in synaptically coupled cell pairs. Excitatory PSPs can initiate prepotentials and action potentials, suggesting dendritic and somatic loci respectively. Neural networks exhibit a broad range of electrophysiologic phenomenology including reciprocal innervation, multiple innervation and synchronous bursting among a widespread population of neurons.