Abstract: AIMTo investigate the effect of benzyltetrahydropalmatine (BTHP) on the rapidly activating component of delayed rectifier K⁺current (I_kr) in single guinea pig ventricular myocytes. METHODSWhole-cell patch clamp technique was used to record I_kr. RESULTS I_kr was blocked by 1～100 μmol·L^-1 BTHP in concentration-, voltage-, and specifically frequency-dependent fashion, with IC₅₀ of 13.5 μmol·L^-1 (95% confidence range: 11.2～15.8 μmol·L^-1). 30 μmol·L^-1 BTHP reduced I_kr and I_kr,tail by (31±4)% and (36±5)% (N=6, P<0.01), respectively. The time constant for deactivation (τ′) of the tail current was decreased by 30 μmol·L^-1 BTHP from (238±16) ms to (196±14) ms, while drug had no any effect on the time constant for activation (τ) of I_kr,tail. CONCLUSIONBTHP inhibited I_kr in a frequency-dependent fashion.