It is well known that cosmological particle production processes may p
henomenologically be described in terms of effective viscous pressures
. We investigate the consistency of this approach on the level of rela
tivistic kinetic theory, using a Boltzmann equation with an additional
source term that leads to a change in the number of gas particles. Fo
r a simple creation rate model we find the conditions for collisional
equilibrium of a Maxwell-Boltzmann gas with increasing particle number
and discuss their cosmological implications. We also comment on the p
ossibility of a bulk pressure driven inflationary phase within causal,
nonequilibrium thermodynamics.