  Cosmological N-body simulations were performed to study the evolution of the
phase-space density Q = rho/sigma^3 of dark matter halos. No significant
differences in the scale relations Q ~ sigma^(-2.1) or Q ~ M^(-0.82) are seen
for "cold" or "warm" dark matter models. The follow up of individual halos from
z = 10 up to the present time indicate the existence of two main evolutionary
phases: an early and fast one (10 > z > 6.5), in which Q decreases on the
average by a factor of 40 as a consequence of the randomization of bulk motions
and a late and long one (6.5 > z > 0), in which Q decreases by a factor of 20
because of mixing induced by merger events. The study of these halos has also
evidenced that rapid and positive variations of the velocity dispersion,
induced by merger episods, are related to a fast decrease of the phase density
Q.
