  We consider a driven Brownian particle, subject to both conservative and
non-conservative applied forces, whose probability evolves according to the
Kramers equation. We derive a general fluctuation relation, expressing the
ratio of the probability of a given Brownian path in phase space with that of
the time-reversed path, in terms of the entropy flux to the heat reservoir.
This fluctuation relation implies those of Seifert, Jarzynski and
Gallavotti-Cohen in different special cases.
