The time decay of fluorescence intensity and anisotropy has been measured for an N-acetylaminoethyl-5-naphthylamine-l-sulfonate probe attached to Cys-98 of troponin C, a dansyl aziridine label linked to Met-25 of troponin C, and 2-p-toluidinyl naphthalene-6- sulfonate bound by calmodulin. In all cases the time decay of intensity at 25° was multi-exponential. The time decay of fluorescence anisotropy was also multi-exponential and could be fitted in terms of a long correlation time reflecting the rotational motion of all, or a major portion of the molecule, and a short correlation time arising from a more localized motion. For the Ca2+-liganded forms of both proteins the magnitude of the longer correlation time could be accounted for in terms of the structure of native calmodulin under these conditions. In the case of the label linked to Cys-98 of troponin C in the absence of Ca2+, the probe senses the rotation of a subelement of the molecule at 25°; at 40° the probe rotates almost independently of the overall structure.