This paper examines the shape information available from measurements of fluorescence anisotropy decays. We report anisotropy decays for three systems and examine the kinds of information which may be obtained. Results for the dye rose bengal (~1000 dal-tons) suggest that an approximation used in rotational diffusion theory, that the solvent molecules are very small compared to the solute, is valid even for this relatively small molecule. Second, we examine the effects of calcium concentration on rotational diffusion of the protein calmodulin (~17,000 daltons) derivatized by crosslinking to form a dityrosine fluorophore. In the presence of sufficiently high calcium ion concentration, this crosslinked calmodulin shows a single exponential anisotropy decay which indicates rotational diffusion consistent with the extended dumbbell structure found for calmodulin by x-ray crystal-lography. At low calcium concentrations, th e crosslinked calmodulin rotates considerably faster, suggesting a much more compact shape; also, this anisotropy decay includes considerably shorter correlation times which are interpreted as arising from a segmental flexibility not evident for crosslinked calmodulin at high calcium concentration. Finally, we examine a transition which is observed at very low salt concentrations for nucleosome core particles, relatively large complexes (~200,000 daltons) derived from chromatin and comprised of eight histone molecules and 145 base pairs of DNA. The anisotropy decay of ethidium bound to the DNA indicates that core particles exposed to low ionic strength are considerably elongated relative to the shape at higher ionic strength.