The study examined the effect of ZD7288, a blocker of hyperpolarization-activated "funny" current If, on electrical activity in working atrial and ventricular myocardium in rats. In concentrations range from 3×10(-6) to 3×10(-5) M, the agent significantly increased the duration of action potentials at 50 and 90% repolarization levels in both atrial and ventricular myocardium at the fixed stimulation rate of 5 Hz. The blocker affected neither resting potential nor the upstroke velocity of action potential. In patch-clamp experiments, ZD7288 selectively inhibited If current, but produced no effect on delayed rectifier potassium currents that determine the rate of repolarization. The described effects of ZD7288 are not related to its non-specific effects on the ionic currents except If.