Two basic issues are addressed: the experimental question of whether it is practicable to generate an instantaneous jump in strain rate to a liquid, and the theoretical question of how a dissolved polymer chain will respond if the solvent around it is suddenly set into motion. It is argued that it is experimentally impossible to perform tests in which the strain rate exhibits a step change. In actual experiments the strain acceleration is bounded and the strain rate as well as the strain vary continuously. It is shown, on the basis of the equation of internal motion for a dumbbell with internal viscosity, that in idealized step strain rate flows the velocities of the beads at the very moment of inception are lower than the flow rates locally imposed on the fluid. In that case, also the mass of the beads affects the strain rate of the dumbbell. Theoretically, the stress jump at the inception of constant strain rate flows depends on φ,φ/f,andφm/f2,where φ is the internal viscosity coefficient,fthe bead‐solvent friction coefficient, andmthe mass of the bead, showing that the usual omission of inertial effects in these models is permitted only ifφ/f<1.