The torsional stress‐strain curves for single‐crystal copper samples were determined at hydrostatic pressures from 1 bar to 15 kbar. The pressure derivative of the yield stress was found to be 3.8×10−3. At strains of about six, the pressure derivative of the flow stress increased to 1.78×10−2. Stress‐strain curves for the various pressures are provided. A simple model of dislocation cell structure is presented in an attempt to explain the experimental results.