  In this paper we examine numerically the Gallavotti-Cohen fluctuation formula
for phase-space contraction rate and entropy production rate fluctuations in
the Nos\'e-Hoover thermostated periodic Lorentz gas. Our results indicate that
while the phase-space contraction rate fluctuations violate the fluctuation
formula near equilibrium states, the entropy production rate fluctuations obey
this formula near and far from equilibrium states as well.
