Many anti-malarial drugs exhibit potent ion channel blocking properties, which can give rise to severe cardiac adverse events. Here we investigate the block of the transient outward K⁺ current (I_to) by the 8-aminochinolone primaquine. Single epicardial myocytes were enzymatically isolated from the left ventricular free wall of female Wistar rats and investigated using the ruptured-patch configuration of the whole-cell patch-clamp technique. At +60 mV, primaquine blocked I_to with an IC₅₀ of 117.68.1mM (n=42). The inactivation time constant of I_to was accelerated dose-dependently and averaged 27.70.7ms (n=39) and 15.40.5ms (n=40) in the absence and in the presence of 30mM primaquine respectively (EC₅₀ = 37.82.8mM, n=26). Steady-state inactivation of I_to was not altered by primaquine whereas I_to recovery from inactivation was prolonged: an additional long time constant appeared, while the short time constant was not altered in its size. The amount of block was use-dependent and block occurred faster at 2Hz than at 0.5Hz. Moreover, it increased with time at depolarized membrane potentials (t=48.37.3ms, n=13). Other K⁺ currents blocked by primaquine were the non-inactivating delayed rectifier K⁺ current (I_SS, IC₅₀=35.02.1mM, n=38) and the inward rectifying K⁺ current (I_K1, IC₅₀=669.5103.7mM, n=22 at -120 mV and 406.964.6mM, n=17 at -60 mV). In Xenopus laevis oocytes, hKv4.2 currents were blocked with an IC₅₀ of 288.830.1mM (n=10). Coexpression of hKChIP2b did not significantly alter the IC₅₀ of the hKv4.2 channels (337.332.2mM, n=8). We conclude that primaquine use-dependently blocks I_to. The properties of block are consistent with open-channel block. Moreover, primaquine is a potent inhibitor of I_SS and only weakly blocks I_K1.