Time-resolved fluorescence measurements at 275 K show that the excited-state lifetime of a model chromophore of the green fluorescent protein (GFP) substantially increases from subpicoseconds in low-viscosity solvents such as ethanol (eta = 1.7 cP) to 30 ps in glycerol (eta = 9.9 x 10(3) cP) and reaches 2.1 ns in glycerol glass at 150 K. At high temperatures the similarity of excited-state decay and ground-state recovery kinetics indicates internal conversion being responsible for the short fluorescence lifetimes. Their viscosity dependence reflects on a motion with a considerable amplitude that is damped by viscous drag and outweighs thermal activation as is concluded from measurements at different temperatures. In solution the neutral and the anionic forms of the model chromophore are similarly nonfluorescent in contrast to wild-type GFP and mutants where the deprotonated form is hardly undergoing internal conversion. Thus, the protein selectively restricts motional degrees of freedom of the chromophore in specific protonation states.