The effect of acidosis on Ca²⁺-activated force generation was studied in rabbit soleus, left ventricular, and adductor magnus muscles. Fibers were skinned (sarcolemma peeled off or mechanico-chemically disrupted) to facilitate direct manipulation and standardization of their intracellular ionic milieus according to bathing solution composition. Skinned single skeletal and small bundles of cardiac fibers were mounted in a photodiode force transducer and activated by immersion in buffered-Ca²⁺ bathing solutions. The magnitude of steady state isometric force at each [Ca²⁺] was determined at pH 7.0 and 6.5 (paired data) at both 1 mM and 10 mM Mg²⁺ in order to detect artifacts of errors in calculated [Ca²⁺]. All bathing solutions contained: 7 mM total EGTA [ethyleneglycol-bis-(-amino-ethylether)-N,N tetra-acetic acid], 70 mM (Na⁺+K⁺), 2 mM MgATP^2– (Mg adenosine triphosphate), 15 mM CP^2– (creatine phosphate), 15 units/ml CPK (creatine phosphokinase), imidazole (adjusted ionic strength to 0.15 M), and propionate anion at 23±1° C. Maximum tensions were similar at both [Mg²⁺]s but less at pH 6.5 than at pH 7.0, with the following order of mean magnitude of acidotic depression adductor>cardiac>soleus. The proportionately greater acidotic depression of submaximum (relative to maximum) forces that occurred only at 1 mM Mg²⁺ (cardiac>adductor>soleus) implicates acidotic depression of Ca²⁺-activated force as a major cause of decreased cardiac contractility.