  In this paper it is shown that Tolman's law can be derived from relativistic
kinetic theory applied to a simple fluid in a BGK-like approximation. Using
this framework, it becomes clear that the contribution of the gravitational
field can be viewed as a cross effect that resembles the so-called
\emph{Thomson effect} in irreversible thermodynamics. A proper generalization
of Tolman's law in an inhomogeneous medium is formally established based on
these grounds.
