Single-channel current-voltage (IV) relations were measured from the calcium release channel of cardiac sarcoplasmic reticulum (RyR2) in 21 mixed solutions of 250 mM alkali metal ions (Na+, K+, and Cs+) with sacroplasmic reticulum lumenal Ca++ ranging from 5 to 50 mM and Mg++ from 1 to 50 mM. The measured IV relations were analyzed by the extended Poisson-Nernst-Planck (PNP) formulation (Chen, D. P.; Xu, L.; Tripathy, A.; Meissner, G.; Eisenberg, B. Biophys. J. 1999, 76, 1346) to give the permeation properties of Call and Mg++. The results indicate that applying PNP theory, two adjustable parameters (diffusion coefficient (D) and "excess" chemical potentials ((μ) over bar)) suffice to predict the flow of Ca++ from the sarcoplasmic reticulum in cardiac muscle. The fitting parameters for Ca++ and Mg++ are D(Ca) = 8.4 +/- 0.7 x10(-8)cm(2)/s, (mu)over bar>(Ca)= -91 +/- 6 mV and D(Mg) = 5.4 +/- 0.7 x 10(-8) cm(2)/s and (μ) over bar (Mg) = -53 +/- 6 mV. The data are used to predict the calcium ion activity profiles in the RyR2 pore, the calcium ion fluxes in solutions of physiological interest, and the competitive permeation of Ca++ and Mg++.