1 The electrophysiological eects of the anti-malarial drug primaquine on cardiac Na + channels were examined in isolated rat ventricular muscle and myocytes. 2 In isolated ventricular muscle, primaquine produced a dose-dependent and reversible depression of dV/dt during the upstroke of the action potential. 3 In ventricular myocytes, primaquine blocked I Na + in a dose-dependent manner, with a K d of 8.2 mM. 4 Primaquine (i) increased the time to peak current, (ii) depressed the slow time constant of I Na + inactivation, and (iii) slowed the fast component for recovery of I Na + from inactivation. 5 Primaquine had no eect on: (i) the shape of the I ± V curve, (ii) the reversal potential for Na + , (iii) the steady-state inactivation and g Na + curves, (iv) the fast time constant of inactivation of I Na + , and (v) the slow component of recovery from inactivation. 6 Block of I Na + by primaquine was use-dependent. Data obtained using a post-rest stimulation protocol suggested that there was no closed channel block of Na + channels by primaquine. 7 These results suggest that primaquine blocks cardiac Na + channels by binding to open channels and unbinding either when channels move between inactivated states or from an inactivated state to a closed state. 8 Cardiotoxicity observed in patients undergoing malaria therapy with aminoquinolines may therefore be due to block of Na + channels, with subsequent disturbances of impulse conductance and contractility.