  Baths produce friction and random forcing on particles suspended in them. The
relation between noise and friction in (generalized) Langevin equations is
usually referred to as the second fluctuation-dissipation theorem. We show what
is the proper nonequilibrium extension, to be applied when the environment is
itself active and driven. In particular we determine the effective Langevin
dynamics of a probe from integrating out a steady nonequilibrium environment.
The friction kernel picks up a frenetic contribution, i.e., involving the
environment's dynamical activity, responsible for the breaking of the standard
Einstein relation.
