We reexamine the multiple-time-scale method as applied to ergodic adia
batic Hamiltonian systems. Solving for the evolution of the phase spac
e density to first order in the slowness parameter, we find a term pre
viously overlooked. The inclusion of this term resolves a standing dis
crepancy between the multiple-time-scale approach to this problem, and
an approach using a Fokker-Planck equation. We apply our solution to
the dynamics of a ''slow'' system coupled to an ensemble of ''fast'' s
ystems following chaotic trajectories.