  We discuss the mechanism of reheating in pre-big bang string cosmology and we
calculate the amount of moduli and gravitinos produced gravitationally and in
scattering processes of the thermal bath. We find that this abundance always
exceeds the limits imposed by big-bang nucleosynthesis, and significant entropy
production is required. The exact amount of entropy needed depends on the
details of the high curvature phase between the dilaton-driven inflationary era
and the radiation era. We show that the domination and decay of the zero-mode
of a modulus field, which could well be the dilaton, or of axions, suffices to
dilute moduli and gravitinos. In this context, baryogenesis can be accomodated
in a simple way via the Affleck-Dine mechanism and in some cases the
Affleck-Dine condensate could provide both the source of entropy and the baryon
asymmetry.
