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1. Voltage clamp studies of the excitatory sodium current, INa, were carried out in rabbit cardiac Purkinje fibres using th two-micro-electrode technique. Previous work has shown the rabbit Purkinje fibre to have relatively simple morphology (Sommer & Johnson, 1968) and electrical structure (Colatsky & Tsien, 1979a) compared to other cardiac preparations. 2. Non-uniformities in membrane potential were kept small by reducing the size of INa to less than 50 microA/cm2 of total membrane surface area through prepulse inactivation or removal of external sodium, Nao. Temporal resolution was improved by cooling to 10-26 degrees C. These adjustments did not greatly alter the measured properties of the sodium channel. 3. Under these conditions, sodium currents were recorded satisfying a number of criteria for adequate voltage control. Direct measurement of longitudinal non-uniformity using a second voltage electrode showed only small deviations at the time of peak current. 4. The properties of the sodium channel were examined using conventional protocols. Both peak sodium permeability, PNa, and steady-state sodium inactivation, h infinity, showed a sigmoidal dependence on membrane potential. PNa rose steeply with small depolarizations, increasing roughly e-fold per 3.2 mV, and reaching half-maximal activation at -30 +/- 2 mV. The h infinity -V curve had a midpoint of -74.9 +/- 2 mV and a reciprocal slope of 4.56 +/- 0.13 mV at temperatures of 10-19.5 degrees C, and showed a dependence on temperature, shifting to more negative potentials with cooling (approximately 3 mV/10 degrees C). Recovery of INa from inactivation in double pulse experiments followed a single exponential time course with time constants of 108-200 msec at 19 degrees C for holding potentials near -80 mV. No attempt was made to describe the activation kinetics because of uncertainties about the early time course of the current. 5. These data predict a maximum duration for INa of less than 1-2 msec and a maximum peak current density of about 500 microA/cm2 under physiological conditions, i.e. 37 degrees C and 150 mM-Nao. This current magnitude is sufficient to discharge the membrane capacitance at rates comparable to those measured experimentally (311 +/- 27 V/sec, Colatsky & Tsien, 1979a). 6. The limitations of the method are discussed. The major problem is the longitudinal cable delay which limits the speed of voltage control. This makes it difficult to separate the activation of INa from the decay of the capacity transient for potentials positive to -15 mV. 7. It is concluded that the approach described is valid for measurements of sodium currents in the potential range where action potentials are initiated, making it possible to study cardiac sodium channels in an adult mammalian preparation which is free of enzymatic treatment.

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