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SUMMARY1. Rapid inward sodium current (INa) was studied on intact rat papillary muscles and trabeculae excised from right or left ventricle using the loose-patch-clamp technique. All experiments were carried out at 25 'C.2. Currents were recorded from patches with a large current density of mean 5-9 + 0 5 mA/cm2. 5. Kinetic data were evaluated using the Hodgkin-Huxley model. 6. Time constants of activation (tm) were estimated using single-pulse and tailcurrent measurements. They had a maximum of about 0-4 ms near the threshold potential and declined at more positive and at more negative potentials to values near 0-1 ms.7. Two time constants were necessary to describe inactivation. Both time constants had their maximal values of 135 + 8-1 and 29-1 + 5-9 ms at about -80 mV and decreased towards 4 and 0-5 ms at potentials positive to -20 mV.

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