High pressure was used to change the viscosity isothermally for the measurement of polarization of fluorescence of small fluorescent molecules in solution. A new technique, necessary for the measurements, as well as a new instrument to perform them, are described and discussed. Although the apparatus is not well suited for determining absolute polarization values, it is quite accurate in detecting changes in polarization due to change in pressure. Independent readings of polarization at 1 atm were therefore obtained using a different instrument. For the sample system, quinine sulfate in isobutanol, there is good agreement between the data obtained by pressure variation and those obtained by temperature variation. The extension of the method to study polymers and proteins in solution and its advantages over previous methods are also discussed.