The purpose of these experiments was to study the pharmacological response of quinidine induced early afterdepolarisations to gain insights into underlying ionic mechanisms. Quinidine (8.5 μM) induced stable early afterdepolarisations at low activation frequencies in 80% of canine cardiac Purkinje fibres superfused with a modified Tyrode's solution. Early afterdepolarisations arose from a secondary plateau in the voltage range of −30 to −60 mV. Calcium channel blockers (verapamil, 1 μM, in 3/6 preparations; verapamil, 10 μM in 6/6 preparations; nifedipine, 0.1 μM in 5/5 preparations) completely eliminated early afterdepolarisations, despite continued quinidine superfusion, without altering the underlying action potential. Isoprenaline (0.2-1 μM) restored them in 75% of these preparations during continued calcium blocker superfusion. Tetrodotoxin (5/5 preparations) eliminated early afterdepolarisations by abbreviating action potentials and reducing or eliminating the quinidine induced secondary plateau. While low concentrations of isoprenaline favoured the occurrence of early afterdepolarisations, larger concentrations eliminated them by enhancing spontaneous automaticity. These experiments suggest that voltage dependent and/or receptor regulated slow inward current plays an important role in quinidine induced early afterdepolarisations. Beta receptor stimulation can enhance or suppress early afterdepolarisations, depending on whether effects on slow inward current (tending to favour them) or on automaticity (suppressing them) predominate.