A recent C-13 NMR study of the rotational dynamics of ATP in viscous solvents suggested that rotation of the adenine moiety about the glycosidic bond that attaches it to the ribose persists at high rates even at high viscosities [Rao, B. D. N.; Ray, B. D. J. Am, Chem. Sec. 1992, 114, 1566-1573]. This is a surprising finding, since it is generally accepted that the rotational diffusion of solutes is attenuated by viscous drag from solvent molecules (according to the classical Stokes-Einstein relationship). In the present study, we have carried out a detailed fluorescence spectroscopic investigation of the rotational diffusion of the adenine in ATP in aqueous solutions containing increasing concentrations of glycerol. For this purpose, we have utilized the well-characterized fluorescent ATP derivative, 1,N-6-ethenoadenosine triphosphate (epsilon-ATP), in which the adenine moiety has been rendered fluorescent. Fluorescence lifetimes and anisotropy decay measurements of epsilon-ATP were performed in solutions presenting viscosities spanning 3 orders of magnitude (ranging from ca. 1.7 to 1310 cp). We have further explored the effect of viscosity on the rotations of the adenine moiety by performing fluorescence anisotropy measurements across the entire excitation band of epsilon-ATP. Selective excitation of epsilon-ATP at different relative orientations between the absorption and emission dipole moments enabled detection of anisotropic rotations of the molecule. Regardless of the direction of the axis selected for excitation of fluorescence, rotations of the adenine moiety in epsilon-ATP were found to be linearly attenuated by increasing solvent viscosity,