  We investigate in detail the parameter space of active-sterile neutrino
oscillations that amplifies neutrino chemical potentials at the epoch of Big
Bang Nucleosynthesis. We calculate the magnitude of the amplification and show
evidences of chaos in the amplification process. We also discuss the
implications of the neutrino chemical potential amplification in the Big Bang
Nucleosynthesis. It is shown that with a $\sim 1$ eV $\nue$, the amplification
of its chemical potential by active-sterile neutrino oscillations can lower the
effective number of neutrino species at Big Bang Nucleosynthesis to
significantly below 3.
