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Continuous low-frequency stimulation (CLFS) by implanted electrodes for 12–24 h led to a significant (∼30%) decrease in the activity of sarcoplasmic reticulum Ca2+-ATPase in fast-twitch extensor digitorum longus (EDL) and tibialis anterior (TA) muscles of intact rats. The decline in catalytic activity after 24 h of CLFS was accompanied by an approximately twofold increase in dinitrophenylhydrazine-reactive carbonyl groups of the enzyme. It also correlated with an immunochemically determined 30% decrease in Ca2+-ATPase protein. Recovery studies after 12 h of CLFS revealed a relatively slow (48–72 h) re-establishment of normal catalytic activity. These findings suggest that the 30% decline of Ca2+-ATPase activity in low-frequency stimulated rat muscle led to an irreversible modification by protein oxidation. The decrease in Ca2+-ATPase protein most likely resulted from the degradation of inactive Ca2+-ATPase molecules. The relatively slow recovery of Ca2+-ATPase activity suggests that de novosynthesis of the enzyme may be necessary to re-attain normal activity.