Dynamic responses of cardiac sodium-calcium exchange current to changes of cytoplasmic calcium and MgATP were monitored and analyzed in giant membrane patches excised from guinea pig myocytes. Secondary dependencies of exchange current on cytoplasmic calcium are accounted for in terms of two mechanisms: (a) The sodium-dependent inactivation process, termed I1 modulation, is itself strongly modulated by cytoplasmic calcium. Recovery from the I1 inactivated state is accelerated by increasing cytoplasmic calcium, and the calculated rate of entrance into I1 inactivation is slowed. (b) A second modulation process, termed I2 modulation, is not sodium dependent. As with I1 modulation, the entrance into I2 inactivation takes place over seconds in the absence of cytoplasmic calcium. The recovery from I2 inactivation is a calcium-dependent transition and is rapid (< 200 ms) in the presence of micromolar free calcium. I1 and I2 modulation can be treated as linear, independent processes to account for most exchange modulation patterns observed: (a) When cytoplasmic calcium is increased or decreased in the presence of high cytoplasmic sodium, outward exchange current turns on or off, respectively, on a time scale of multiple seconds. (b) When sodium is applied in the absence of cytoplasmic calcium, no outward current is activated. However, the full outward current is activated within solution switch time when cytoplasmic calcium is applied together with sodium. (c) The calcium dependence of peak outward current attained upon application of cytoplasmic sodium is shifted by approximately 1 log unit to lower concentrations from the calcium dependence of steady-state exchange current. (d) The time course of outward current decay upon decreasing cytoplasmic calcium becomes more rapid as calcium is reduced into the submicromolar range. (e) Under nearly all conditions, the time courses of current decay during application of cytoplasmic sodium and/or removal of cytoplasmic calcium are well fit by single exponentials. Both of the modulation processes are evidently affected by MgATP. Similar to the effects of cytoplasmic calcium, MgATP slows the entrance into I1 inactivation and accelerates the recovery from inactivation. MgATP additionally slows the decay of outward exchange current upon removal of cytoplasmic calcium by 2-10-fold, indicative of an effect on I2 inactivation. Finally, the effects of cytoplasmic calcium on sodium-calcium exchange current are reconstructed in simulations of the I1 and I2 modulation processes as independent reactions.