  A billiard in the form of a stadium with periodically perturbed boundary is
considered. Two types of such billiards are studied: stadium with strong
chaotic properties and a near-rectangle billiard. Phase portraits of such
billiards are investigated. In the phase plane areas corresponding to decrease
and increase of the velocity of billiard particles are found. Average
velocities of the particle ensemble as functions of the number of collisions
are obtained.
