Small-to-moderate-angle X-ray scattering studies were performed on a troponin C solution prepared from rabbit skeletal muscle. The main feature of the scattering profile is the existence of two regions in the Guinier plot, in which data points lie on a straight line. The scattering profile was analyzed by way of a structural model with 'dumbbell' shape, based on the structure given by X-ray crystallography on troponin C. The dumbbell model shows a `quasi-' straight-line region at higher scattering angles in the Guinier plot, in addition to the intrinsic straight-line region at very small scattering angles. A ln[I(S)/S] versus S2 plot of the scattering profile at higher scattering angles gives information as to the average radius of gyration of the N-terminal and C-terminal domains of troponin C. The molecular parameters determined from small-angle X-ray scattering data of troponin C (10 mg ml−1), in the absence of Ca2+ ions, are as follows: the radius of gyration of the whole troponin C molecule is 20.4 A and the average radius of gyration of the N- and C-terminal domains is 15.6 A. From these radii of gyration, the center-to-center distance between the two domains is estimated to be about 26 A.