3,5,3´-triiodothyronine (T3), regulates energy metabolism by oxygen consumption and thermogenesis via induction of NaK-ATPase in vivo. These changes are correlated with nuclear T3 binding in responsive organs. We showed that creatine kinase (CK) another ATP regulating enzyme, is modulated by different hormones, in tissues containing appropriate receptors. This modulation is via binding to specific receptors and changes in CK mRNA levels. Since CK is involved in energy metabolism, we examined T3 effects on CK activity in tissues containing T3 receptors, such as kidney (Ki), cerebrum (cx), cerebellum (cbl), epiphyseal cartilage (Ep) and diaphyseal bone (Di). In hypothyroid rats, basal CK activity increased in different tissues and the normal developmental pattern of CK was partially maintained. T3 injection into hypothyroid rats lowered CK activity in Ki, cx and Di but increased it in cbl and Ep. Hormonal injection into euthyroid rats resulted in increased CK only in cbl and Ep. Parallel changes were detected in CK mRNA expression and stimulation of DNA synthesis in the different organs. Adenylate kinase, another energy metabolising enzyme did not change by thyroid status or treatment. These results suggest that T3 modulates CK activity both directly and indirectly via changes in ATP levels. In different organs except cbl and Ep, changes in CK may be compensatory to changes in NaK-ATPase. In Ep and cbl where T3 increases cell proliferation, it may also induce CK directly since we have shown its correlation with stimulation of cell proliferation.