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Fractional changes in the peak sodium conductances of the cardiac cell membrane during the action potential are often estimated from fractional changes in the minimum time derivative of the action potential upstroke (V max). The present model study shows that this approach is valid for propagating action potentials provided that the membrane capacitance does not change and that the nonsodium current is small at the time of V max. When the maximum conductance of the sodium channel (g Na) and the sodium equilibrium potential (E Na) are varied independently of one another, fractional changes in either of them can be predicted from fractional changes in V max if a reasonable estimate of the initial value of E Na is available. It is concluded that properly designed experimental approaches based on V max measurements can yield important information on manipulations affecting g Na, E Na, and h infinity (V) during propagation, and that a better accuracy is possible in cardiac muscle when measurements are made during transverse propagation.