Ranolazine attenuates cardiac arrhythmic activity associated with hypoxia and hydrogen peroxide (H 2 O 2) by inhibition of late sodium current (late I Na). The mechanism of ranolazine's action on Na+ channels was investigated using whole-cell and single-channel recording from guinea pig isolated ventricular myocytes. Hypoxia increased whole-cell late I Na from− 0.48±0.02 to− 3.99±0.07 pA/pF. Ranolazine at 3 and 9 μmol/L reduced the hypoxia-induced late I Na by 16%±3% and 55%±3%, respectively. Hypoxia increased the mean open probability and open time of Na+-channel late openings from 0.016±0.001 to 0.064±0.007 milliseconds and from 0.693±0.043 to 1.081±0.098 milliseconds, respectively. Ranolazine at 3 and 9 μmol/L attenuated the hypoxia-induced increase of open probability by 19%±7% and 61%±1%, and increase of open time by 26%±19% and 74±21%, respectively. H 2 O 2 increased the mean open probability and open time of Na+-channel late openings from 0.013±0.002 to 0.107±0.015 milliseconds and from 0.689±0.075 to 1.487±0.072 milliseconds, respectively. Ranolazine at 3 and 6 μmol/L reduced the H 2 O 2-induced increase of mean open probability by 60%±7% and 95%±2%, and the increase of mean open time by 31%±21% and 82%±8%. In conclusion, the inhibition by ranolazine of hypoxia-and H 2 O 2-stimulated late I Na is due to reduction of both the open probability and open time of Na+-channel late openings.