This review summarizes estimates for cytoplasmic-free concentrations of Ca²⁺ ([Ca²⁺]_i) and Mg²⁺ ([Mg²⁺]_i) at rest and during contraction of skeletal muscles, from which substantial quantitative information about them has been accumulated. Although the estimates of resting [Ca²⁺]_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 [Ca²⁺]_i of approximately 0.05-0.1 μM are likely to be correct. Following action potential propagation, the Ca²⁺ release from the sarcoplasmic reticulum causes a transient rise of [Ca²⁺]_i (Ca²⁺ transient). The large peak amplitude and brief time course of the Ca²⁺ transients have been established only recently by studies with low-affinity Ca²⁺ indicators developed in the past decade. These technical improvements in [Ca²⁺]_i measurements have made it possible to study relationships between [Ca²⁺]_i and force in intact muscle fibers. in the second part of this review, various estimates of [Mg²⁺]_i in the resting muscle are discussed. Relatively recent estimates of the [Mg²⁺]_i level appear to be about 1.0 mM. Using the current knowledge of concentrations and reaction properties of intracellular Ca²⁺ -Mg²⁺ binding sites, we constructed a model for dynamic Mg²⁺ movement following Ca²⁺ transients. The model predicts that with a train of action potentials, the sustained rise of [Ca²⁺]_i produces an elevation of [Mg²⁺]_i of about 220 μM.