We describe a new mammalian expression system that optimizes conditions for the measurement of Na channel gating currents (Ig). The small magnitude of Ig limits their study to preparations with high numbers of Na channels to improve signal-to-noise ratios. To increase Na channel Ig signals, single tsA201 cells (approximately 20 microns in diameter) were fused into large, multinucleated cells by treatment with polyethylene glycol. After being placed in cell culture for 48-72 h, fused tsA201 cells develop a spherical geometry with diameters up to 200 microns. Because of the large plasma membrane surface area, fused tsA201 cells are able to express high levels of Na channels after transient transfection with Na channel cDNAs using Lipofectamine. Typically, 5 days after transfection, fused tsA201 cells that are 60-100 microns in diameter are selected for voltage clamp with a large suction pipette (a pore size of 20-30 microns) that allows for both a low series resistance and internal perfusion. Approximately two-thirds of transfected fused tsA201 cells express Na current, with nearly one-third of transfected cells expressing sufficient numbers of Na channels to allow for the ready measurement of Ig. In addition to fused tsA201 cells being a preparation well suited for the study of Ig, they should also be useful for measurement of electrical signals from other voltage-gated channels and transporters that generate small electrical signals.