A computer model was used to calculate the shape and average velocity v of a dislocation acted on by a viscous drag, and moving through a field of point obstacles. As a special case, the operation of a Frank‐Read source was computed also. At sufficiently low velocities (and stress levels), the behavior is governed entirely by the obstacle spacing and strength; conventional tensile testing probably falls in this regime. At sufficiently high velocities (and stress levels), the behavior is governed entirely by the viscous drag; shock deformation falls in this regime. In between lies a wide range in which the two effects superimpose, each contributing measurably to the flow stress. The strain rate sensitivity m = (∂lnv∕∂lnσ)T, where σ is the stress, reflects this behavior. At low and high velocities, respectively, m characterizes the obstacles and the drag. But in between m is characteristic of neither, and in fact reflects the change in the contributions of the two strengthening mechanisms when the applied stress is changed.