The reactions of hydrated electrons produced during pulse radiolysis have been utilized to investigate the binding of ethidium bromide to heparin. Complexes of ethidium bromide and heparin can be dissociated with salt. Divalent cations are more effective than monovalent cations in this respect. Pulse-radiolysis investigations at different temperatures indicate that the thermodynamic parameters governing the interaction of ethidium bromide with heparin are deltaH' = 11-6 kcal mole-1 and deltaS' = 42-6 cal deg-1 mole-1.