The effect of cytosolic Mg²⁺ on halothane-induced Ca²⁺ release from the sarcoplasmic reticulum (SR) was investigated in mechanically skinned fibres from the rat extensor digitorum longus (EDL) muscle. Preparations were perfused with solutions mimicking the intracellular milieu and changes in [Ca²⁺] were detected using Fura-2 fluorescence. In the presence of 1 mM Mg²⁺, brief (500 ms) applications of 40 mM halothane failed to induce Ca²⁺ release from the SR. However, Ca²⁺ release became detectable when [Mg²⁺] was reduced to 0.4 mM, and the amplitude of the response increased progressively as [Mg²⁺] was further reduced to 0.2 and 0.1 mM. Lower halothane concentrations within the range found during anaesthesia or induction (0.1-1.2 mM) failed to induce SR Ca²⁺ release at 0.2 or 0.4 mM Mg²⁺. However, in further experiments, preparations were exposed to 1 mM halothane for 2-3 min under conditions where the volume of solution surrounding the preparation was restricted by stopping the flow. In the absence of perfusion, 1 mM halothane induced Ca²⁺ release from the SR at 0.4 mM Mg²⁺ in two out of six preparations, and release was observed consistently at 0.2 and 0.1 mM Mg²⁺. Responses to 1 mM halothane induced in the presence of 0.4 and 0.2 mM Mg²⁺ were typically delayed in onset and involved a localised release of Ca²⁺ that propagated along the fibre. These results suggest that halothane-induced Ca²⁺ release is strongly inhibited at normal physiological levels of Mg²⁺. However, when Mg²⁺-induced inhibition of the ryanodine receptor (RYR) is reduced, levels of halothane within the range found during anaesthesia can induce a marked efflux of Ca²⁺ from the SR. This may be of relevance to the condition of malignant hyperthermia, where the inhibition of RYRs by Mg²⁺ is reportedly reduced.