Experiments were performed on several plastics under constant hydrostatic pressures up to 50,000 psi. Measurement of linear strain indicates that the volume decreases with time at constant pressure in accordance with a power function of time. The exponent of time was about the same as observed for tension or torsion creep of the same material. Recovery upon abrupt release of pressure was nearly instantaneous, except for the thermal effect resulting from decompression. Partial release of pressure resulted in a time-dependent recovery. The instantaneous recovery was explained in terms of the opening up of the polymer structure on release of pressure.