  We study dynamical generation of entanglement in bipartite quantum systems,
characterized by purity (or linear entropy), and caused by the coupling between
the two subsystems. Explicit semiclassical theory of purity decay is derived
for integrable classical dynamics of the uncoupled system, and for localized
(general Gaussian wave-packet) initial states. Purity decays as an algebraic
function of time times strength of perturbation, independently of the Planck's
constant.
