Adaptations of the kinetic properties of mitochondria in striated muscle lacking cytosolic (M) and⁄or mitochondrial (Mi) creatine kinase (CK) iso-forms in comparison to wild-type (WT) were investigated in vitro. Intact mitochondria were isolated from heart and gastrocnemius muscle of WT and single- and double CK-knock-out mice strains (cytosolic (M-CK –⁄ – ), mitochondrial (Mi-CK –⁄ – ) and double knock-out (MiM-CK –⁄ – ), respect-ively).