This review summarizes estimates for cytoplasmic-free concentrations of Ca
2+ ([Ca
2+]
i) and Mg
2+ ([Mg
2+]
i) at rest and during contraction of skeletal muscles, from which substantial quantitative information about them has been accumulated. Although the estimates of resting [Ca
2+]
i in the literature widely differ, which is because of the variety of difficulties related to different methodologies used, recent studies suggest that estimates of resting [Ca
2+]
i of approximately 0.05-0.1 μM are likely to be correct. Following action potential propagation, the Ca
2+ release from the sarcoplasmic reticulum causes a transient rise of [Ca
2+]
i (Ca
2+ transient). The large peak amplitude and brief time course of the Ca
2+ transients have been established only recently by studies with low-affinity Ca
2+ indicators developed in the past decade. These technical improvements in [Ca
2+]
i measurements have made it possible to study relationships between [Ca
2+]
i and force in intact muscle fibers. in the second part of this review, various estimates of [Mg
2+]
i in the resting muscle are discussed. Relatively recent estimates of the [Mg
2+]
i level appear to be about 1.0 mM. Using the current knowledge of concentrations and reaction properties of intracellular Ca
2+ -Mg
2+ binding sites, we constructed a model for dynamic Mg
2+ movement following Ca
2+ transients. The model predicts that with a train of action potentials, the sustained rise of [Ca
2+]
i produces an elevation of [Mg
2+]
i of about 220 μM.
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