1. Force development and 45Ca movement between the internal membranes and the myofilament space were measured in segments of skinned muscle fibres.2. At 20° C calcium ions in the myofilament space of calcium loaded fibres caused the internal membranes to release a substantial amount of calcium which produced a large force spike. Calcium produced a similar force spike in fibres containing only the physiological level of calcium when the temperature was lowered to about 6° C.3. The force spike at 6° C was inhibited when the magnesium concentration in the bathing solution was increased from 1 to 6 mM. The pattern of force development in preparations transferred directly from oil to a free calcium solution indicated that the normal concentration of Mg2+ in the myofilament space of intact fibres is of the order of 10-3 M.4. A possible role for the regenerative calcium release mechanism in the physiological activation of muscle fibres is suggested.
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