  We consider the physics of neutrinos in a fireball, i.e. a tightly coupled
plasma of photons, positrons and electrons. Such a fireball is believed to form
in the first stages of a gamma-ray burst. We assume the fireball is
radiation-dominated and spherically symmetric. Energy considerations limit the
allowed baryon density, from which it follows that the neutrino physics is
dominated by leptonic processes. We find that, for quite general initial
conditions, neutrinos start out in thermodynamic equilibrium with the fireball
and follow the usual hydrodynamical evolution. As the fireball cools, the
plasma becomes transparent to neutrinos which subsequently decouple from the
plasma. Although a sizable fraction of the total energy is carried away, the
detection possibility of these neutrino bursts is limited due to the isotropic
outflow and the relatively low mean energy of approximately 60 MeV.
