The conformation of fibrinogen in solution has been investigated by steady-state fluorescence polarization measurements. Factor XIIIa has been employed to enzymatically incorporate 1-6 mol of dansylcadaverine/mol of fibrinogen into a specific glutamine residue near the carboxy terminus of the gamma chain and up to two sites on the alpha chain. The fluorescence emission maximum of the labeled protein is shifted to 495 nm (from 538 nm for the fluorophore in solution) and the intensity substantially enhanced, indicating the covalently linked dansyl groups residue in a hydrophobic environment in the interior of the protein. This covalent modification does not interfere with the formation of fibrin, following thrombin activation. Steady-state fluorescence polarization measurements were carried out as a function of temperature and in high viscosity solvents. The fluorescent lifetime of dansylcadaverine-fibrinogen was determined by a phase shift technique. Analysis of the data by the Perrin-Weber treatment yields a rotational relaxation time of 160 ns, considerably faster than any realistic hydrodynamic model of fibrinogen would predict. The results are discussed in terms of segmental flexibility.