1. The effects of decreasing pH from 7.40 to 6.20 on the tension developed by direct activation of the myofilaments and by Ca2+ release from the sarcoplasmic reticulum were studied comparatively in segments of single cells of skeletal muscle (frog semitendinosus) and cardiac muscle (rat ventricle) from which the sarcolemma had been removed by micro-dissection (skinned muscle cells). 2. The concentration of free Ca2+ in the solutions was buffered with ethylene glycol-bis (beta-aminoethylether N,N'-tetraacetic acid (EGTA). The change of the buffer capacity of a given [total EGTA] caused by varying pH and the uncertainty about the value of the equilibrium constant for Ca-EGTA have been taken into account in the interpretation of the results. 3. Decreasing pH from 7.40 to 6.20 produced an increase in the [free Ca2+] required for the myofilaments to develop 50% of the maximum tension by a factor of about 5 in skinned cardiac cells but of only 3 in skeletal muscle fibres. In addition, acidosis depressed the maximum tension developed in the presence of a saturating [free Ca2+] by approximately the same amount in the two tissues. 4. The pH optimum for loading the sarcoplasmic reticulum of skinned fibres from skeletal muscle decreased when the pCa (-log [free Ca2+]) in the loading solution decreased. The optimum was pH 7.40-7.00 for a loading at pCa 7.75, pH 7.00-6.60 at pCa 7.00 and pH 6.60-6.20 at pCa 6.00. 5. The pH optimum for loading the sarcoplasmic reticulum of skinned cardiac cells with a solution at pCa 7.75 was about pH 7.40 as in skeletal muscle fibres. But the cardiac sarcoplasmic reticulum could not be loaded with a [free Ca2+] much higher than pCa 7.75 because a higher [free Ca2+] triggered a Ca2+-induced release of Ca2+ from the sarcoplasmic reticulum. 6. The pH optimum of about 7.40 for the loading of the cardiac sarcoplasmic reticulum was also optimum for the Ca2+-induced release of Ca2+ from it. 7. It was concluded that the effects of acidosis on the cardiac sarcoplasmic reticulum accentuate the depressive action of decreasing pH on the myofilaments. This may explain the pronounced depression of contractility observed during acidosis in cardiac muscle. In contrast, a moderate acidosis causes an effect on skeletal muscle sarcoplasmic reticulum that could compensate for the depressive action on the myofilaments, which is, in addition, less pronounced than in cardiac muscle.