Hydrostatic pressure effects on the cyclization of polystyrene chains (M(n) = 2600, M(w)/M(n) = 1.07; M(n) = 4500, M(w)/M(n) = 1.08), both ends labeled with 1-pyrenyl groups,were studied by measuring fluorescence spectra and pyrene monomer and excimer decay curves. The experiments were performed at room temperature in toluene (good solvent), cyclopentane (Theta solvent), and methylcyclohexane (poor solvent), covering a pressure range of 1-3500 bar. The cyclization rate constant decreases monotonically with pressure, as a result of the increase of solvent viscosity. However, changes in the quality of solvent for the polymer induced by pressure also influence chain cyclization. Activation volumes for chain cyclization compare well with solvent viscosity activation volumes when solvent quality to the polymer is pressure independent. The rate of excimer dissociation and ring-opening increases with pressure, especially in Theta and poor solvents. This effect is ascribed to the lowering of the excimer binding energy, induced by changes on the excimer conformation.