Proton magnetic resonance spectroscopy has been used to study the cation(Mg2+, Ca2+)‐dependent conformational states of the C‐terminal domain of rabbit skeletal troponin C under a variety of solution conditions. Nuclear Overhauser data and paramagnetic probe observations provide definition of the configuration of this region of troponin C. Comparative study of homologous proteins identify common features of the tertiary structure relevant to the cation binding reaction. Complex formation with troponin I and the drug trifuloperazine is observed to adjust the solution conformation of the C‐terminal domain of troponin C. The interactive conformational response to cation coordination and of the binding of the drug and troponin I are discussed.