The mechanical behavior of OFHC copper at strain rates from 10−3to 103sec−1at 300°, 420°, and 590°K was investigated. The strain rate behavior of copper can be divided into two regions. Below 10 sec−1the dislocation motion is thermally activated over forest dislocation barriers. Above 103sec−1, a linear relationship between stress and strain rate was observed indicating the presence of a viscous damping mechanism. The stress level &tgr;Bthat must be exceeded in order to obtain viscous behavior depends on the work‐hardened state of the copper. The mobile dislocation density in the viscous damping region was found to be (1) independent of strain rate, (b) only a small fraction (10−5) of the total dislocation density, (c) independent of strain, and (d) increased with increasing temperature. These deductions are discussed in terms of the dislocation multiplication and annihilation mechanism.