The structure of polyethylenimine in aqueous and ethanolic solutions was investigated by measuring the fluorescence polarization of dansyl conjugates. In aqueous solutions, all polyethylenimines, whatever their molecular weight, are associated into large aggregates containing rotating units having rotary relaxation times corresponding to spherical particles of molecular weight about 3–6 × 103. The interaction of polyethylenimine with very dilute solutions of linear high polymers has been investigated by fluorescence polarization measurements and by the measurement of turbulent drag. Addition of polyethylenimine causes the solutions of anionic polymers to lose their capability of reducing turbulent drag. The results are discussed in relation to the proposed structure of the anionic–cationic complexes formed in the mixtures.