Using whole-cell patch-clamp techniques in situ (whole-tissue and tissue slices), we have studied two aspects of rat telencephalic cell development during the period of embryogenesis starting at E12. The first aspect was related to junctional coupling as revealed by low input resistance, intercellular dye spread and pharmacologic blockade. Coupling appeared to decrease with time, both in extent and occurrence. The second aspect dealt with cell excitability as revealed by voltage-dependent Na current (INa) expression. Immature action potentials and their underlying INaS were present in a small proportion of E12 cells. These currents were blocked 36% and 78% by 10(-7) M and 10(-6) M tetrodotoxin (TTX), respectively. From then onward, INaS got larger and more prevalent while no obvious changes in kinetics were observed. At E21, INaS were abolished by 10(-7) M TTX and channel density apparently was sufficient to support overshooting yet still immature action potentials.