  The Lyapunov exponents and the KS entropy for a two dimensional Lorentz gas
at low densities are defined for general non-equilibrium states and calculated
with the use of a Lorentz-Boltzmann type equation. In equilibrium the density
dependence of these quantities predicted by Krylov is recovered and explicit
expressions are obtained. The relationship between the KS entropy, Lyapunov
exponents, and escape rate, conjectured by Gaspard and Nicolis for
non-equilibrium systems is confirmed and generalized.
