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1. Sarcoplasmic reticulum fragments (SRF) and actomyosin were isolated from skeletal muscle of cats treated either with thyroxine or with placebo tablets, for 5-16 months.

2. Ca²⁺ uptake and binding by SRF, measured in the presence and absence of oxalate respectively, were reduced by thyroxine treatment.

3. Actomyosin from the thyroxine-treated animals underwent ATP-induced syneresis at a faster rate than did that from the controls.

4. Syneresis of the actomyosin preparations from controls and treated animals was inhibited to the same extent by EGTA when rates of syneresis were made the same by adjustment of the KCl concentration in the assay media. In contrast, at any given KCl concentration, syneresis of actomyosin from thyroxine-treated animals was inhibited to a lesser extent by EGTA.

5. At the end of the isolation procedure, the amount of Ca²⁺ remaining in the actomyosin suspension was similar for both treated and control animals.

6. It was concluded that the effect of thyroxine on skeletal muscle may be the result of an action on both the sarcoplasmic reticulum and the contractile protein complex.