The cellular and synaptic physiology of developing rat neocortical neurons was studied using the in vitro slice method. Rats aged 1-28 days were used for analysis. During the first two postnatal weeks several sequential changes occur in membrane properties and evoked synaptic potentials. Immature neurons had higher input resistances, more linear I-V characteristics, longer membrane time constants, and slower rising and falling phases of action potentials. The developmental increase in rate of rise of the action potential suggests an increasing density of voltage-dependent Na+-channels are inserted in neuronal membranes during postnatal development. The higher input resistance of young cells might be due to their small size and differences in membrane properties. The long time constant indicates a higher specific membrane resistivity of immature neurons. Postsynaptic potentials (PSPs) recorded in young neurons were longer in latency, longer in duration, and more fragile during repetitive activation than their mature counterparts. In addition, PSPs evoked in neurons of animals less than 1 week old did not contain inhibitory postsynaptic components. These physiological features of immature neocortical neurons help explain the pattern of epileptogenesis in young animals. When neonatal cortical slices were exposed to the gamma-aminobutyric acid (GABA) antagonists penicillin or bicuculline, the frequency of occurrence of discharges resembling epileptiform depolarization shifts approached that found in mature slices only during the second postnatal week. Depolarization shifts at younger ages were less stereotyped and more sensitive to stimulus parameters than those in mature neurons.