It is unclear whether chromanol 293B, a selective inhibitor of slow component of delayed rectifier K + current (I K s ), may affect other K + currents in human atrium. With whole-cell patch configuration, we evaluated effects of 293B on transient outward K + current (I t o 1 ) and ultra-rapid delayed rectifier K + current (I K u r ) in isolated human atrial myocytes. It was found that 293B inhibited I t o 1 and I K u r in a concentration-dependent manner. At 10 μM 293B suppressed I t o 1 to 3.4 +/- 0.4 from 5.1 +/- 0.3 pA/pF (P < 0.01), and I K u r to 1.5 +/- 0.2 from 2.1 +/- 0.3 pA/pF (P < 0.01) at +50 mV. The inhibition of I t o 1 and I K u r was independent of depolarizing voltage, and the concentration of 50% inhibition was 31.2 μM for I t o 1 , and 30.9 μM for I K u r . 293B blocked I t o 1 and I K u r with the same concentration range, and the significant effect was observed from the concentration of 1 μM. The maximum inhibitive effect was 88% for I t o 1 and 96% for I K u r at 250 μM. Voltage dependence of activation and inactivation, and time-dependent recovery from inactivation of I t o 1 were not altered by 293B; however, time to peak and time-dependent inactivation of I t o 1 was significantly accelerated. The results indicate that 293B significantly inhibits the major repolarization K + currents I t o 1 and I K u r in human atrial myocytes.