  We discuss how the first order Langevin equation for the overdamped dynamics
of an interacting system has a natural time reversal of simple but surprising
form, with consequences for correlation functions. This leads to the
correlation of interactions as a strictly restraining term in the
time-dependent diffusion tensor of the system, deriving the relation first
suggested by Fixman. Applying this to the time-dependent diffusion of dilute
polymer coils leads to the quantitative calibration of Kirkwood's approximation
for their hydrodynamic radius. We find the generalized ``Fixman Law" for
dissipation with a memory kernel, which has revealing causal structure, and we
also discuss the case of the second order Langevin Equation.
