  We give the explicit structure of the functional governing the dynamical
density and current fluctuations for a mesoscopic system in a nonequilibrium
steady state. Its canonical form determines a generalised Onsager-Machlup
theory. We assume that the system is described as a Markov jump process
satisfying a local detailed balance condition such as typical for stochastic
lattice gases and for chemical networks. We identify the entropy current and
the traffic between the mesoscopic states as extra terms in the fluctuation
functional with respect to the equilibrium dynamics. The density and current
fluctuations are coupled in general, except close to equilibrium where their
decoupling explains the validity of entropy production principles.
