  The influence of mesonic fluctuations on quantities in the Nambu-Jona-Lasinio
model is examined in different approximation schemes: an expansion in powers of
1/N_c and an expansion up to one-meson loop in the effective action formalism.
It is explicitely proved that the Goldstone theorem as well as the
Gell-Mann-Oakes-Renner relation hold within those schemes. The influence of
meson-loop effects on the quark condensate, the pion mass, the pion decay
constant and properties of rho- and sigma-meson are investigated. First we
focus on the determination of a consistent set of parameters. In the 1/N_c
expansion scheme it is possible to find a set of parameters which allows to
simultaneously describe the quantities in the pion sector and those related to
the rho-meson, whereas this turns out to be not possible within the expansion
of the effective action. Besides, the relation of our model to hadronic models
is discussed. In the last part of this thesis the behavior of the quark
condensate at nonzero temperature is studied. In the low-temperature region
agreement with the model independent chiral perturbation theory result in
lowest order can be obtained. The perturbative 1/N_c expansion scheme does not
allow for an examination of the chiral phase transition at higher temperatures,
whereas this is possible within the meson-loop approximation scheme. A first
order phase transition is found.
