With a new method called "Action Potential-Clamp" (APC) we studied in single SA nodal cells the contribution of both transient and long lasting calcium currents (L-type and T-type) to the action potential. Action potentials were recorded by a computer and a representative cycle was subsequently repetitively replayed to the same cell under voltage clamp. Blockade of the L-type calcium current (D600) or T-type calcium current (nickel) revealed the quantitative and time related contributions of these currents to the action potential, since the blocked current is compensated by the clamp amplifier.