In order to study the effects of BmkTXKβ on transient outward potassium current(I_to) of isolated rabbit atrial myocytes，standard whole-cell patch clamp technique was used to record I_to before and after administration of extracellular BmkTXKβ with multiple concentrations. The results showed that: (1) At a dose of 1 μmol/L, it decreased I_to by 41.4% (n=16, P＜0.001) at membrane potential of +50 mV ［from (13.63±0.87) pA/pF to (7.98±0.78) pA/pF］. After washout, I_to restored to (11.18±0.82) pA/pF (n=6, P＜0.01). (2) It significantly reduced I_to in a clearly concentration-dependent manner at the range of 0.01～100μmol/L with an IC₅₀ value of 0.95 μmol/L (n=10, P＜0.01), but without any change in frequency-dependence (n=6，P＞0.05). (3) In the absence and presence of BmkTXKβ (1 μmol/L),the activation curves from relative conductance almost overlapped, whereas steady-state inactivation curve shifted to left from （－23.6±2.7） mV to （－35.3±3.6） mV at V_1/2 point significantly （n=8，P＜0.05）. The time for 50% recovery delayed obviously from（51.2±8.5） ms to （93.5±13.4） ms in the absence and presence of 1 μmol/L BmkTXKβ（n=9，P＜0.01）. The results show that BmkTXKβ exerts direct blocking effect on I_to in rabbit atrial myocytes, which is mainly caused by a strong suppression effect on inactivation duration and prolongation of the recovery duration from inavtivation.