The results of studies on modulation of Na channel function are often difficult to interpret due to time-dependent changes in channel kinetics. Although the "tight-seal" whole cell voltage-clamp technique has proved very useful in studying the properties of the cardiac Na current, the spontaneous shift of parameters of inactivation and activation gating to more negative potential is a serious limitation to the use of the technique. The shifts are believed to result from changes in the intracellular milieu effected by dialysis; moreover, use of a variety of different anions and cations in the internal micropipette solution has not obviated the problem. The perforated-patch technique permits low-resistance intracellular access without free dialysis between the intracellular solution and the recording micropipette. We have compared steady-state inactivation and peak current-voltage relationship of whole cell Na currents measured with the conventional whole cell and perforated-patch techniques in rabbit atrial myocytes at 17 degrees C. Although gating parameters shifted to more negative potentials when recorded with the conventional technique, stable kinetics could be observed for up to 150 min with the perforated-patch technique. The potential for one-half Na channel inactivation was -73 +/- 5.1 mV and is consistent with measurements made using indirect techniques such as upstroke velocity measurements. The fact that the intracellular milieu is left relatively intact makes the approach attractive for studying modulation of the Na current by neurotransmitters and hormones.