We have measured the fluorescence spectrum of ethidium bromide dissolved in both glycerol and water and intercalated into DNA following excitation by a 30-ps laser pulse. The streak camera based spectrometer allows approximately 20-ps time resolution. Time-dependent spectral changes of excited singlet ethidium bromide in water and DNA are very small, but ethidium bromide in glycerol shows a large red shift which is nonexponential in time and occurs on a subnanosecond time scale. This change can be attributed in part to phenyl group rotation with respect to the phenanthridium ring in the excited state. Such a reaction may not be activated but forced by the bond torsion against the viscous solvent. The fast rate and low activation energy of this reaction suggest that it is not controlled by the static viscosity of the glycerol solution, but by the high-frequency component of the frictional forces of the viscous solvent. © 1986 American Chemical Society.