1. A persistent inward current activated by depolarization was recorded using the whole‐cell, tight seal technique in rat isolated cardiac myocytes. The amplitude of the inward current increased when cells were exposed to a solution with low oxygen tension. 2. The persistent inward current had the characteristics of the persistent Na+ current described previously in rat ventricular myocytes: it was activated at negative potentials (‐70 mV), reversed close to the equilibrium potential for Na+ (ENa), was blocked by TTX and was resistant to inactivation. 3. Persistent single Na+ channel currents activated by long (200‐400 ms) depolarizations were recorded in cell‐attached patches on isolated ventricular myocytes. Hypoxia increased the frequency of opening of the persistent Na+ channels. 4. Persistent Na+ channels recorded during hypoxia had characteristics similar to those of persistent Na+ channels recorded at normal oxygen tensions. They had a null potential at ENa, their amplitude varied with [Na+], they were resistant to inactivation and their mean open time increased with increasing depolarization. 5. The persistent Na+ channels in cell‐attached patches were blocked by TTX (50 microM) in the patch pipette and by lidocaine (100 microM). 6. It was concluded that hypoxia increases the open probability of TTX‐sensitive, inactivation‐resistant Na+ channels. The voltage dependence of these channels, and their greatly increased activity during hypoxia, suggest that they may play an important role in the generation of arrhythmias during hypoxia.